arXiv:0812.2561v1 [astro-ph] 13 Dec 2008 & npress in A&A Universit¨ats-Sternwa und Institut Astrophysikalisches ..b ao ta.20,Rgaa ta.20,o otre- most or 2006, al. . et 2007) Barbieri Raghavan & 2006, Desidera reporte by al. cently (as et to Bakos used host by are e.g. 2MASS of or companions POSS new like identify surveys visib as sky from well data all addition, as infrared In and 2007), 2007). al. al. et et (Luhman al. Neuh¨auser space Eggenberger et from recently 2006, & most Luhman al. and (2002), et 2007, al. Chauvin et (2002), Patience Jayawardhana e.g. (see di observations 2004b contrast limited 2004a, AO high al. seeing 2007b), et 2007a, Mugrauer with 2006a, e.g. 2005b, out (see carried imaging multiplici infrared were ongoing near com- studies by primary brighter These identified mor the systems, studies. be stellar to recent of out during turned ponent but them of stars, more single and isolated h exoplanet are these of stars majority sta The -like neighborhood. three solar mostly the than around in revolve more that identified indirectly hundred have pla campaigns photometric and search radial-velocity decade, a than more For Introduction 1. rat multiplicity the Hence, known. presently are 43 which of 7.-09A,080.C-0312(A) 079.C-0099(A), eetls fcnre iayadtil trssesta h that systems triple and Methods. binary confirmed of list recent Aims. jena.de Conclusions. follow- with determined are Results. types spectral and whose (0.18 rmr.I otat D220 sacoeM wr (0.35 dwarf M3 close a is B 212301 HD contrast, In primary. eto t primary. its of west edo Send coe 6 2018 26, October ⋆ srnm Astrophysics & Astronomy ae nosrain bando aSlai S programs ESO in Silla La on obtained observations on Based M epeetnwrslsfo u non utpiiysuyo study multiplicity ongoing our from results new present We ⊙ ff opno fteeolnths trH 262 oae abo located 125612, HD star host exoplanet the of companion ) efudtonwc-oigcmain fteeolnths s host exoplanet the of companions co-moving new two found We rn eussto requests print euedrc mgn oietf ieselradsubstella and stellar wide identify to imaging direct use We h iaisH 262A n D220 Baenwmmesi th in members new are AB 212301 HD and AB 125612 HD binaries The e o-asselrcmain fteeolnths stars host exoplanet the of companions stellar low- New h utpiiyo xpae otstars host exoplanet of multiplicity The aksMgae,[email protected] Mugrauer, Markus : aucitn.1 no. manuscript D151 n D212301 HD and 125612 HD .Mugrauer M. ff ato limited raction t ea cilr¨ßhn23 74 ea Germany Jena, 07745 Schillerg¨aßchen 2-3, Jena, rte ro exoplanets. arbor feolnths tr saot1 %. 17 about is stars host exoplanet of e ppooer n spectroscopy. and photometry up M ⊙ ABSTRACT ,wihi on bu . rsc( arcsec 4.4 about found is which ), net ost 1 rs, ty le d e n .Neuh¨auser R. and , opnosa omvn bet oteosre exoplanet observed the to objects co-moving as companions r xpae otsas are u ihSofI with out carried stars, host exoplanet f ytm rsnl nw sH 51 A these 65216 HD of closest is The known summary). presently be a triple to for systems hierarchical 2007a, out AU. are 10000 al. turned systems than et themselves Mugrauer these more companions i.e. to the binaries, up cases close sep- tens few projected few a with a In stars between sequence of main arations low-mass are stars zto ftecoeteolnths iais ieG 6AB 86 Gl like or binaries, 2006), host al. character- et exoplanet dynamical Lagrange closest (see 2007). a the surveys al. et of imaging Liu these ization and was to 2006b, B, addition al. 3651 HD In et dwarf Mugrauer brown a (see T7-T8 of discovered the companion star, substellar imaged host B, directly 86 exoplanet Gl first (e.g. the on, star Later nearby a of survive ∼ evolution can sequence exoplanets ex- main that of post 2007b). suggesting companions as stars, al. identified et host (Mugrauer been oplanet have AU 250 dwarfs white about Also of separation jected oeg urure l 20c rBnvt eiea(2007) Desidera & Bonavita refe or we (2007c) studies al. et statistical Mugrauer recent e.g. For to evo orbits. the and their of of tion process formation the on multiplicity their of set full the determine elements. to bital out carried being is 2007), t15aci ( arcmin 1.5 ut 0A,seMgae ehasr20,frmr details). more for Neuh¨auser 2005, & Mugrauer see AU, 20 asH 262adH 131 D151 sawd 4dwarf M4 wide a is B 125612 HD 212301. HD and 125612 HD tars oto h eetdselrcmain feolnthost exoplanet of companions stellar detected the of Most l hs e these All otnosygoigls feolnths trsystems star host exoplanet of list growing continuously e ff ∼ rswl ept eeltetu mato stellar of impact true the reveal to help will orts 1 70A fpoetdsprto)suhes fits of south-east separation) projected of AU 4750 ∼ 3 Uo rjce eaain north- separation) projected of AU 230 γ e B(e ehasre al. Neuh¨auser et (see AB Cep ⋆ / T.W rvd h most the provide We NTT. + C ihapro- a with BC,

c otstars, host S 2018 ESO (see s here r the or- lu- n . 2 M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars

In this paper we present new results of our ongoing multi- plicity study carried out at with SofI/NTT. We detected two new low-mass stellar companions of the ex- oplanet host stars HD125612 and HD212301. Our SofI astro- and photometry is described in section2, the follow-up spec- troscopy in section3. The properties of the newly found exo- host binaries are described in section4, where the SofI detection limits also are presented and the separation space of possible, so far undetected, additional companionsis discussed. A list of all presently known and confirmed exoplanet host star systems is given in the Appendix.

2. Astro- and photometry 2.1. HD 125612 The exoplanet host star HD125612 is a solar-like G3V star (1.1±0.07 M⊙, Fischer et al. 2007), which can be foundon the sky between the and Libra at a distance of 53±4pc (Hipparcos, Perryman & ESA 1997). According to Fischer et al. (2007) HD125612 shows only weak chromo- ′ spheric activity (logR = −4.85), and its chromospheric age HK Fig. 1. SofI large field image of the exoplanet host star of 3.3 ± 2Gyr is comparable with the age determined with evo- HD125612. The detected co-moving companion of the exo- lutionary models (0.16 to 5.6Gyr). In our ongoing multiplic- planet host star is indicated with a black arrow. ity study of exoplanet host stars, HD125612 was observed the first time in June 2007 with SofI/NTT. All our SofI images are astrometrically calibrated with the 2MASS point source cata- logue (Skrutskie et al. 2006), as summarized in Table1.

Table 1. The astrometrical calibration of SofI/NTT. The pixel scale PS and the detector position angle PA with their uncer- tainties are listed. The detector is tilted by PA from north to west.

instrument PS [′′] PA [◦]

SofIlarge 06/2007 0.28791±0.00021 89.984±0.026 SofIlarge 01/2008 0.28794±0.00025 89.997±0.056

Our reduced SofI H-band image of HD125612 is shown in Fig. 1. The total integration time of this image is 10 min taken with the standard set-up of our programme (for further details Fig. 2. The derived motion of all companion candidates de- see e.g. Mugrauer et al. 2007a). Several faint companion can- tected in our SofI H-band image and also imaged by 2MASS didates down to H∼18mag (S/N = 10) are detected around the around the exoplanet host star. The proper and parallactic mo- exoplanet host star, which is located in the center of the image. tion of HD125612 for the give epoch difference is indicated HD125612 was imaged by 2MASS in April 1998, i.e more with a black square. than 9 years before our SofI 1st epoch observation. HD125612 exhibits a high (µRA = −64.47 ± 1.57mas/yr µDec = −65.64 ± 1.03mas/yr) which is well known from candidate located about 89.98arcsec (∼4750AU of projected Hipparcos. By comparing the 2MASS with our SofI image we separation) south-east of HD125612 clearly shares the proper derive the proper motion of all companion candidates detected motion of HD125612. This newly found co-moving compan- in our SofI, and also in the less sensitive 2MASS image (see ion, indicated with a black arrow in Fig.1, will be denoted Fig. 2). as HD125612B and the exoplanet host star as HD125612A, The majority of detected objects only exhibits small or neg- from here on. ligible proper motion. Hence, all these candidates are unrelated We also measure the separation and the position angle of background objects only randomly located close to the line of HD125612B relative to its primary in the 2MASS, as well sight in the direction of the exoplanet host star. In contrast, one as in our SofI H-band image. This relative of the M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars 3 companion is summarized in Table2. Neither separation nor 2pc, (Hipparcos, Perryman & ESA 1997) in the position angle of HD125612B change significantly during the . more than 9years of epoch difference, as is expected for a co- We observed HD212301 twice with SofI in H-band, with moving companion. a total integration time of 10min in June 2007 as well as in January 2008. Our 1st epoch SofI image is shown in = Table 2. The separations and position angles of HD125612B Fig.3. Several faint companion candidates, down to H 18mag = and HD212301B relative to their primaries — the exoplanet (S/N 10), are detected around the bright exoplanet host star. host stars HD125612A, and HD212301A. In the columns We could also identify a close candidate, located in both SofI images only about 4.38arcsec (∼ 230AU of projected separa- sep i f bg and PA i f bg we show the expected separation and po- sition angle in the case that both companions are non-moving tion) north-west of the HD212301. background objects.

epoch sep obs[arcsec] sep if bg [arcsec] HD 125612 B 2MA 04/98 89.957±0.055 89.957±0.055 NTT 06/07 89.994±0.066 89.574±0.056 HD 212301 B 2MA 12/99 4.34±0.23 4.34±0.23 NTT 06/07 4.36±0.06 5.05±0.23 NTT 01/08 4.43±0.06 5.08±0.23 ◦ ◦ epoch PA [ ] PA if bg [ ] HD 125612 B 2MA 04/98 162.696±0.070 162.696±0.070 NTT 06/07 162.682±0.052 162.205±0.071 HD 212301 B 2MA 12/99 275.7±3.0 275.7±3.0 NTT 06/07 275.1±0.5 283.0±3.0 NTT 01/08 275.8±0.5 283.3±3.0 Fig.3. Left: Our 1st epoch SofI H-band image of the exoplanet host star HD212301. Right: Detail of the whole SofI image, showing the central region around the exoplanethost star, using The proper motion of HD125612B is also listed in the a logarithmic scaling. A close companion candidate is detected only 4arcsec north-west of HD212301, marked with a black USNO-B1.0 catalogue (Monet et al. 2003) (µRA = −64 ± arrow. 1mas/yr, µDec = −58±4mas/yr) and is fully consistent with the Hipparcos proper motion of the exoplanet host star. This is fur- ther proof of the astrometric companionship of HD125612B In order to check if this close candidate is also detected by to the exoplanet host star. 2MASS, we carefully inspected the 2MASS J-, H-, and KS- We determine the H-band photometry of HD125612B band images. We found that in all 2MASS images the PSF in our SofI image and obtain H=11.761±0.034mag, of HD212301 appears elongated in the direction where the which is fully consistent with its 2MASS photometry close candidate is located. In contrast, the PSF of objects found H=11.773±0.023mag. In addition, the 2MASS point around the star in the 2MASS images all exhibit radially sym- source catalogue also lists the J-, and KS-band photom- metric PSFs. We deconvolved all 2MASS images, using the etry of HD125612B (J = 12.381 ± 0.0244mag, and PSFs of objects detected around HD212301 as a reference. KS = 11.514 ± 0.024mag). The well-known Hipparcos The object functions of the exoplanet host star in the J- and distance of the exoplanet host star, and the given 2MASS KS-band remain elongated, while in the H-band the two sepa- apparent magnitudes of HD125612B, finally yield the abso- rated object functions of HD212301 and its close companion lute photometry of the companion: MJ = 8.77 ± 0.16mag, candidate could be reconstructed.

MH = 8.16 ± 0.16mag, MKs = 7.90 ± 0.16mag. By comparing our two SofI images of HD212301 with According to the evolutionary models for low-mass stars the deconvolved 2MASS H-band image taken at the end of from Baraffe et al. (1998) the derived absolute photometry of 1999, we can determine the proper motion of all detected ob- HD125612B is consistent with a 0.184 ± 0.012 M⊙ star for an jects. Due to the high proper motion of HD212301 (µRA = assumed age between 1 and 5 Gyr. Accordingto the magnitude- 76.11 ± 0.85mas/yr and µDec = −91.64 ± 0.63mas/yr, from spectral type relation from Reid et al. (2004), the absolute pho- Hipparcos) and the long epoch difference of more than seven tometry of HD125612B is consistent with an M4 dwarf. The years between the 2MASS and our SofI images, real compan- spectral type estimation has to be confirmed by follow-up spec- ions of HD212301 can easily be identified as co-moving ob- troscopy (see next section). jects. The derived proper motions of all candidates between the 2MASS and our 2nd epoch SofI observation are shown in Fig. 4. 2.2. HD 212301 Only the close companion candidate clearly shares the The exoplanet host star HD212301 is a 1.9 to 5.4Gyr proper motion of the exoplanet host star. Hence, this is a new (Holmbergetal. 2007) old F8 dwarf (1.27 ± 0.02 M⊙, co-moving companion of HD212301, called HD212301B Lo Curto et al. 2006), which is located at a distance of 53 ± from here on. 4 M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars

spectral type relation from Reid et al. (2004) we expect the spectral type of the companion to be M3V, which has to be confirmed with follow-up spectroscopy (see the following sec- tion).

3. Spectroscopy In June 2007, we obtained H- and K-band follow-up spec- troscopy of the two co-moving companions HD125612B and HD212301B with SofI. We used the grism RED in com- bination with a 1 arcsec slit which offers a resolving power λ/∆λ = 588 and dispersion of 10.22Å per pixel. For both co- moving companions we always took 10 frames each with an integration time of 60s. Between individual exposures a nod- ding of 45arcsec between two positions along the slit, as well as a 5arcsec random jitter was applied. For wavelength cali- Fig. 4. The derived proper motion of all companion candidates bration we took spectra of a Xenon lamp. Standard IRAF rou- detected around the exoplanet host star HD212301 in our 2nd tines for spectroscopy were used for data reduction. Telluric epoch SofI, and the deconvolved 2MASS H-band image. The features in the reduced spectra were removed by dividing with proper and parallactic motion of the exoplanet host star for the spectra of telluric standard stars, whose spectra were always give epoch difference is indicated with a black square. taken directly after the spectroscopy of the companions, with airmass difference between science and calibration spectra of less than 0.1. We took spectra of HIP73881 (B2-3V) in the Beside the proper motion of HD212301B we also deter- case of HD125612B, and HIP100170 (B2V) for HIP212301, mined its relative astrometry to its primary in all observing respectively. The spectral response function of SofI was deter- epochs, summarized in Table2. The separation and position an- mined using the spectra of the telluric standards, as well as flux- gle of HD212301B do not change significantly over time, as calibrated spectra from the spectral library of Pickles (1998). expected for a co-moving companion. The flux-calibrated H- and K-band spectra of HD125612B Because of its small angular separation HD212301B is not and HD212301B are shown in Fig.5 and Fig.6. The spectra resolved in the 2MASS J, and KS-band images. Therefore, we of both companions are compared with template spectra from obtained additional photometry with SofI in June 2007 and ob- the IRTF spectral library (Cushing et al. 2005), smoothed to the served HD212301B in J, and KS broad, as well as in the Brγ same resolution as our SofI spectra (∆λ/λ = 1/588). narrow band filter. Within a jitter box of 20arcsecs we obtained In the H-band spectrum of HD212301B the most promi- three images in J and KS and five images in Brγ which are each nent features are those of aluminium at 1.674 µm, magnesium the average of 50 exposures with an integration time of 1.2s, at 1.711 µm and weaker at 1.576 µm, potassium at 1.517 µm, and obtained J = 11.056 ± 0.086mag, and Ks = 10.210 ± and the faintly detected silicon line at 1.589 µm. All detected 0.058mag. In the H-band we obtained H = 10.516±0.041mag spectral features, as well as the continuum shape of the spec- in our 1st, and H = 10.587 ± 0.053mag in the 2nd SofI trum compare well with the M3V template spectrum. In con- imaging epoch. The KS band magnitude of the exoplanet host trast, the continuum of the spectrum of HD125612B is more star is listed in the 2MASS point source catalogue (KS = consistent with that of an M5 dwarf. In addition the potassium 6.466 ± 0.021mag). Together with our SofI KS-band photome- line in the spectrum of HD125612B is stronger than that of try of HD212301B this yields a magnitude difference between the spectrum of HD212301B, which indicates a slightly later HD212301A and B of ∆KS = 3.744±0.061mag. This result is spectral type of between M3 and M5V. confirmed by the magnitude difference between HD212301B In the K-band, the spectra of both companions show the and the exoplanet host star ∆Brγ = 3.67 ± 0.06mag, as mea- prominent absorption line doublet of sodium at 2.208 µm, and sured in our SofI Brγ image. In addition, we also determined the molecular absorption bands of CO at wavelengths longer the photometryof HD212301B in the deconvolved2MASS H- than 2.294 µm, typical of M3 to M5 dwarfs. In contrast, the band image and obtained H2Mass = 10.71 ± 0.17mag which is calcium absorption line doublet at 2.265 µm which is well de- consistent with our SofI H-band photometry of the co-moving tected in the spectrum of HD212301B cannot be identified companion. in the spectrum of HD125612B. Because all atomic absorp- With the measured apparent photometry of HD212301B, tion features weaken to later spectral types this indicates that and the well known distance of the exoplanet host star we the spectrum of HD125612B, is slightly later than that of derive its absolute photometry, which is MJ = 7.45 ± 0.12, HD212301B mostly consistent with a M4 to M5 dwarf.

MH = 6.94 ± 0.09, and MKs = 6.60 ± 0.10. According to our SofI spectroscopy we can conclude that With the evolutionary models of Baraffe et al. (1998) and HD125126B and HD212301B are both mid M dwarfs (M3 the derived absolute photometry of HD212301B we determine to M5V). Our SofI spectroscopy fully confirms the spectral the mass of the companionto be 0.35±0.02 M⊙, for an assumed type estimation of both companions obtained from their appar- age of between 0.5 and 5Gyr. According to the magnitude- ent photometry, assuming that they are dwarfs located at the M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars 5

Fig. 5. The H-band SofI spectra of HD125612B and HD212301B together with comparison spectra of dwarfs from Fig. 6. The K-band SofI spectra of HD125612B and the IRTF Spectral Library (Cushing et al. 2005). The most HD212301B together with comparison spectra of dwarfs from prominentspectral atomic and molecular features are indicated. the IRTF spectral library (Cushing et al. (2005)). The most prominentspectral atomic and molecular features are indicated. distances of the exoplanet host stars (M4V for HD125612B, and M3V for HD212301B). Hence, the companionship of HD125612B and HD212301B, which was first revealed with astrometry (common proper motion), is finally confirmed by photometry and spectroscopy.

4. Discussion As described in detail in the last sections HD125612B is a wide M4 dwarf companion with a mass of ∼0.18 M⊙ which is separated by about 4750AU from its primary, the exoplanet host star HD125612A. In contrast, HD212301B is a close M3 dwarf (0.35 M⊙) companion, located at a separation of about 230AU from its primary. The typical SofI detection limit for both exoplanet host stars is shown in Fig.7. At an angular separation of less than 1.5arcsec (∼80AU) saturation occurs, i.e. companions can- not be detected in this region. According to the evolution- ary models from Baraffe et al. (1998) all stellar companions Fig. 7. The average detection limit (S/N = 10) in our (mass > 0.078 MJup) are detectable beyond6arcsec (∼320AU) SofI images of the exoplanet host stars HD125612A and around both stars, for assumed ages between 1 and 5Gyr. In the HD212301A plotted for a range of angular separations up to background limited region beyond about 15arcsec (∼ 800 AU) 25arcsec. The detected co-moving companion HD212301B is a sensitivity of H=18mag (S/N = 10) is reached, i.e. compan- indicated as a black square. ions with absolute magnitudes down to MH = 14.4mag can be imaged in this region. This allows the detection of brown dwarf companions with masses down to 37 to 65 MJup. Companions due to bad weather. For HD212301 we obtained a 2nd epoch with angular separations of up to about 143arcsec (∼ 7500AU image in January 2008 but only to confirm the detection of the of projected separation) can be detected together with the exo- companion HD212301B and to rule out that it is a fast moving planet host stars in our SofI images. foreground object e.g. a planetoid in the solar system. The ex- HD125612was observedonly in the 1st epochwith SofI. A pected total motion of the companion between both observing 2nd epoch follow-up imaging could not be carried out in 2008 epochs is only about 40mas, too small to be clearly detected 6 M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars with SofI in its large field mode. Hence, follow up SofI imag- A.1. Exoplanet host systems ing is needed for both stars with sufficient epoch difference to 55CncAB1, 83LeoBA, GJ3021AB, GJ777AB, Gl86AB, check the companionship of all faint companion candidates not HD 114762 AB, HD 142 AB, HD 195019 AB, υ And AB, imaged by 2MASS. HD 222582 AB, HD 27442 AB, HD 46375 AB, τ Boo AB, Nevertheless, we can already exclude additional compan- HD 80606 AB, HD 41004 AB, HD 19994 AB, HD 11964 AB (for ions around both exoplanet host stars based on the 2MASS de- details see e.g. Mugrauer et al. 2007a); HD 147513 AB (Mayor et al. tection limit. According to Skrutskie et al. (2006) the 2MASS 2004); HD 75289 AB (Mugrauer et al. 2004a); HD 89744 AB H-band limit is 15.1mag at S/N = 10, the limit for all de- (Mugrauer et al. 2004b); HD 16141 AB, HD 114729 AB, and tected sources with accurate astrometry. As determined by HD 213240 AB (all described in detail in Mugrauer et al. 2005); us, this limit is reached in the 2MASS images beyond about HD 38529 AB, and HD 188015 AB (both reported by Raghavan et al. 20arcsec (∼1060AU) around both stars. According to the evo- 2006); HD 189733 AB (Bakos et al. 2006); HD 142022 AB lutionary models from Baraffe et al. (1998) and an assumed (Eggenberger et al. 2006); HD 196885 AB (Chauvin et al. 2007); HD 20782 AB, and HD 109749 AB (both described by age of 1 to 5Gyr companions with masses down to 63 and Desidera & Barbieri 2007); γ Cep AB (whose B component was 74 MJup can be detected in 2MASS. All stellar companions directly detected first by Neuh¨auser et al. 2007, who also clarified the (mass > 0.078 MJup) can be detected beyond about 13arcsec true nature of this component); HD 101903 AB (Mugrauer et al. (∼690AU) around both stars. 2007b); HD 177830 AB (Eggenberger et al. 2007); ADS 16402 BA In addition, we can also rule out companions that should (Bakos et al. 2007); HD 156846 AB (Tamuz et al. 2008); not be on long-term stable orbits around the exoplanet host HD 125612 AB, and HD 212301 AB (both presented in this work) stars in both binary systems. According to Holman & Wiegert (1999) only companions with semi-major axes smaller than the critical semi-major axis exhibit long-term stable orbits. A.2. Exoplanet host triple star systems In the case of the HD125612AB system (HD125612A HD 178911 B+AC, HD 40979 A+BC, 16CygB+AC, and (∼1.1 M⊙), HD125612B (∼0.18 M⊙), 4750AU of projected HD 219449 A+BC (see Mugrauer et al. 2007a, for details); separation) we expect the long-term stable zone to extend up HD 196050 A+BC (see Mugrauer et al. 2005b and Eggenberger et al. to ∼650AU (12.4arcsec of angular separation), assuming that 2007); HD 65216 A+BC (Mugrauer et al. 2007b) HD125612B is on a circular orbit, whose semi-major axis cor- responds to the observed projected separation of the compan- References ion. This value can be considered as an upper separation limit for additional long-term stable companions. In the case of an Bakos, G. A.,´ P´al, A., Latham, D. W., Noyes, R. W., & Stefanik, R. P. eccentric orbit of HD125612B the extent of this long-term sta- 2006, ApJ, 641, L57 ´ ble zone should be smaller. Indeed, there is one faint compan- Bakos, G. A., et al. 2007, ApJ, 656, 552 ion candidate in our SofI image detected about 10arcsec west Baraffe, I., Chabrier, G., Allard, F., & Hauschildt, P. H. 1998, A&A, 337, 403 of HD125612A. Follow-up imaging is needed to test the com- Bonavita, M., & Desidera, S. 2007, A&A, 468, 721 panionship of this faint companion candidate. Chauvin, G., Lagrange, A.-M., Udry, S., Fusco, T., Galland, F., Naef, In the case of the HD212301AB system (primary mass of D., Beuzit, J.-L., & Mayor, M. 2006, A&A, 456, 1165 about 1.27 M⊙, secondary mass of ∼ 0.35 M⊙, with a projected Chauvin, G., Lagrange, A.-M., Udry, S., & Mayor, M. 2007, A&A, separation of ∼ 230AU) we derive an extent of the long-term 475, 723 stable zone of only ∼ 38AU, i.e about 0.7arcsec of angularsep- Cushing, M. C., Rayner, J. T., & Vacca, W. D. 2005, ApJ, 623, 1115 aration. No objects are detected within this given angular radius Desidera, S., & Barbieri, M. 2007, A&A, 462, 345 around HD212301A in our SofI images. Eggenberger, A., Mayor, M., Naef, D., Pepe, F., Queloz, D., Santos, The binaries HD125612AB and HD212301AB are two N. C., Udry, S., & Lovis, C. 2006, A&A, 447, 1159 new members of the growing list of exoplanet host multiple Eggenberger, A., Udry, S., Chauvin, G., Beuzit, J.-L., Lagrange, A.- star systems. Today 250 exoplanet host stars are known and M., S´egransan, D., & Mayor, M. 2007, A&A, 474, 273 Fischer, D. A., et al. 2007, ApJ, 669, 1336 ongoing multiplicity studies so far have found 43 of them to Holman, M. J., & Wiegert, P. A. 1999, AJ, 117, 621 be components of a multiple star system (see the Appendix for Holmberg, J., Nordstr¨om, B., & Andersen, J. 2007, A&A, 475, 519 summary). Hence, the multiplicity-rate of the exoplanet host Lagrange, A.-M., Beust, H., Udry, S., Chauvin, G., & Mayor, M. 2006, stars is at least 17 %. A&A, 459, 955 Liu, M. C., Leggett, S. K., & Chiu, K. 2007, ApJ, 660, 1507 Acknowledgements. We would like to thank the technical staff of the Lo Curto, G., et al. 2006, A&A, 451, 345 ESO NTT. We made use of the 2MASS public data releases as well as Luhman, K. L., & Jayawardhana, R. 2002, ApJ, 566, 1132 the Simbad database operated at the Observatoire Strasbourg. Luhman, K. L., et al. 2007, ApJ, 654, 570 Mayor, M., Udry, S., Naef, D., Pepe, F., Queloz, D., Santos, N. C., & Appendix A: Presently known exoplanet host star Burnet, M. 2004, A&A, 415, 391 systems Monet, D. G., et al. 2003, AJ, 125, 984 Mugrauer, M., Neuh¨auser, R., Mazeh, T., Alves, J., & Guenther, E. Currently, 43 confirmed star systems are known that harbor at least 2004a, A&A, 425, 249 one exoplanet; 37 of them are binaries, and the remaining 6 are triples. There is only one system known with a directly detected wide substel- 1 For all exoplanet host star systems listed, the first component is lar companion, namely HD 3651 AB (Mugrauer et al. 2006b). always the exoplanet host star, the second one is its stellar companion. M. Mugrauer and R. Neuh¨auser: The multiplicity of exoplanet host stars 7

Mugrauer, M., Neuh¨auser, R., Mazeh, T., Guenther, E., & Fern´andez, M. 2004b, Astronomische Nachrichten, 325, 718 Mugrauer, M., & Neuh¨auser, R. 2005, MNRAS, 361, L15 Mugrauer, M., Neuh¨auser, R., Seifahrt, A., Mazeh, T., & Guenther, E. 2005, A&A, 440, 1051 Mugrauer, M., Neuh¨auser, R., Mazeh, T., Guenther, E., Fern´andez, M., & Broeg, C. 2006a, Astronomische Nachrichten, 327, 321 Mugrauer, M., Seifahrt, A., Neuh¨auser, R., & Mazeh, T. 2006b, MNRAS, 373, L31 Mugrauer, M., Neuh¨auser, R., & Mazeh, T. 2007a, A&A, 469, 755 Mugrauer, M., Seifahrt, A., & Neuh¨auser, R. 2007b, MNRAS, 378, 1328 Mugrauer, M., 2007c, PhD Thesis, University of Jena, ThULB, 39.40 Sternsysteme, Diss 2007-13 Neuh¨auser, R., Mugrauer, M., Fukagawa, M., Torres, G., & Schmidt, T. 2007, A&A, 462, 777 Patience, J., et al. 2002, ApJ, 581, 654 Perryman, M. A. C., & ESA 1997, ESA Special Publication, 1200, Pickles, A. J. 1998, PASP, 110, 863 Raghavan, D., Henry, T. J., Mason, B. D., Subasavage, J. P., Jao, W.- C., Beaulieu, T. D., & Hambly, N. C. 2006, ApJ, 646, 523 Reid, I. N., et al. 2004, AJ, 128, 463 Skrutskie, M. F., et al. 2006, AJ, 131, 1163 Tamuz, O., et al. 2008, A&A, 480, L33