arXiv:1801.06036v1 [astro-ph.SR] 18 Jan 2018 ye(innik17) nAtp ytm,temr massive more the systems, and A-type A-type In curves: light 1970). typ their main (Binnendijk to two type according are th There objects day. of these a periods of than orbital shorter The usually are components. objects both ge of heavy the distortion of ric because eclipses, of outside even variation, binary of kind such studying lacking, for still motivation systems. is further issues give the Nevertheless all may spectroscopy. resolves from that obtained model dif o general one photometry lot from the a determined from are ratio questi fers there mass open that the some is leaves where one obvious systems it most , expla The these can 2010). of it (Rucinski and properties simple of very lot is a e model same 1968), this the nearly (Lucy Although have envelope perature. they compo- why convective reason The common the is point. a which Lagrangian share physical (L1) also making inner nents and the lobes through Roche contact their bot co model, filling their accepted around most are the orbiting components to are According that sta mass. type of sequence spectral center main K mon or two G of F, the consists of typically binary contact A Introduction 1. aur 9 2018 19, January Astronomy .Mitnyan T. h ih uvso otc iaisso otnosflux continuous show binaries contact of curves light The e words. Cep. Key VW of variation da period available the the for entirely, explanation out cau likely Applegate-mechanism ruled be the can and mechanisms system, varia these the period in the body explain fourth To activity. a the sec of the occ nature phenomenon than observed flip-flop active that more propose We is activity. component mospheric primary the that confirm Results. Methods. Aims. Context. eevd.. cetd... accepted ...; Received vial htcnepaneeyosre rpryo hss this of property observed every explain can that available apndi h ytmi h attodcds n opropose to and decades, two past the in system the in happened qiaetwdh fteH the of widths equivalent ftesse.Rda eoiiso h opnnswr dete were components the of velocities Radial system. the of ailvlcte eemdle iutnosywt the with simultaneously modelled were velocities radial 4 3 2 1 h otc iayV ehirvstd ufc ciiyan activity surface revisited: Cephei VW binary contact The aaAtooia bevtr fUiest fSee,H-6 Szeged, of e-mail: University of Observatory Astronomical Hungary Baja Ear 15-17, and út Miklós Astronomy Thege for Konkoly Centre Research Observatory, Konkoly eateto xeietlPyis nvriyo Szeged, of University Physics, Experimental of Department eateto pisadQatmEetois nvriyo University Electronics, Quantum and Optics of Department u oiaini ooti e pcr ln ihphotometr with along spectra new obtain to is motivation Our & 1 ept h atta WCpe soeo h elsuidcon well-studied the of one is Cephei VW that fact the Despite ehv edtrie h hsclprmtr ftesystem the of parameters physical the re-determined have We .Bódi A. , [email protected] o h eidaayi edtrie 0nwtmso iiaf minima of times new 10 determined we analysis period the For Astrophysics tr:atvt tr:idvda:V e iais clos binaries: – Cep VW individual: stars: – activity stars: 2 , 3 .Szalai T. , aucitn.Mitnyan_etal_VWCep_rev2 no. manuscript α iewr esrdo hi di their on measured were line 1 .Vinkó J. , 1 eidvariation period , ff 3 .Szatmáry K. , cietem- ective PHOEBE which omet- ystem. ff ino WCp ets w rvosysgetdseais p scenarios: suggested previously two test we Cep, VW of tion asgetta astase ihasol eraigrt g rate decreasing slowly a with transfer mass that suggest ta ABSTRACT erences. ons ese W- rigo h rmr opnn ol eapsil explanat possible a be could component primary the on urring m- a , es in rs -70See,Dmtr9 Hungary 9, tér Dóm Szeged, H-6720 e yproi antcatvt.W ocueta althoug that conclude We activity. magnetic periodic by sed h 0 aa zgd t t 6,Hungary 766, Kt. út, Szegedi Baja, 500 mnduigtecoscreaintcnqe h ih cu light The technique. cross-correlation the using rmined zgd -70See,Dmtr9 Hungary 9, tér Dóm Szeged, H-6720 Szeged, f e da o xliigthem. explaining for ideas new Pál - f nay n hr sacreainbtensotdesadt and spottedness between correlation a is there and ondary, oe l bevdsetawr oprdt ytei spect synthetic to compared were spectra observed All code. hSine,HnainAaeyo cecs -11Budapes H-1121 Sciences, of Academy Hungarian Sciences, th 3 neo natvt yl iial owa a ese nour on seen be can what to similarly cycle pre . activity the the indicate an of may of evolution phenomenon ence and This motion regions. the active of cooler because mag to the orbit of from di manifestation a The as activity. components netic the of surface the on e (O’Connell maxima 1974). (Rucinski sta reason secondary unknown the an that for of assumed temperature curves is higher light a it the model, explain this to in order binaries; in commo literature also the model, in secondary used hot Nevert the 1975). scenario, tha (Mullan another than secondary less, lower cooler and be massive to less appears the brightness spott surface heavily averaged mass so its more is the component that primary is explan hotter objects W-type intrinsically accepted of most existence the The for brightness. tion surface higher seem component show secondary to the ra where mass systems, W-type smaller se the and the than periods than orbital longer brightness have surface They ondary. higher has component primary 95 ekr rwy19;Oá tasee 2002; Strassmeier al. & et Oláh Baliunas 1978; 1995; Wilson (e.g. Ordway & stars Heckert binary 1995; and single of 2 cmaueet,t nlz htkn fcagsmyhave may changes of kind what analyze to measurements, ic .Borkovits T. , atbnre nteltrtr,teei oflyconsisten fully no is there literature, the in binaries tact iais cisn tr:starspots stars: – eclipsing binaries: – e ciiycce aebe ietyosre nsvrltype several on observed directly been have cycles Activity curve light the in asymmetry show binaries contact of lot A codn oornwlgtcreadseta oes We models. spectral and curve light new our to according o u ih uvs n osrce e O new a constructed and curves, light our rom 3 , 4 .I Bíró I. B. , ff ff rnebtentemxm a change can maxima the between erence c) hc slkl asdb starspots by caused likely is which ect), 4 .Hegedüs T. , ril ubr ae1o 13 of 1 page number, Article vstemost the ives 4 − .Vida K. , diagram C eec of resence oeof none h o fthe of ion vsand rves

echro- he c model t aand ra S 2018 ESO d t, dthat ed 3 A. , these has r tios ive, of t nly he- se c- a- s- s s - A&A proofs: manuscript no. Mitnyan_etal_VWCep_rev2

Vida et al. 2015). In the cases of contact binaries there is only terpreted as a consequence of enhanced chromospheric activity. indirect evidence for periodic magnetic activity. These mostly They pointed out a possible anticorrelation between the photo- come from period analysis, which have revealed that the peri- spheric and chromospheric activity. The latest publication about odic variation between the observed and calculated moments of VW Cephei including spectroscopic analysis is a Doppler imag- mid-eclipses cannot be simply explained with the presence of ing study by Hendry & Mochnacki (2000). They derived a mass additional components in the system (or, it can only be a par- ratio of q = 0.395 0.016 and found that the surfaces of both tial explanation). Applegate (1992) proposed a mechanism that componentswere heavily± spotted, and the more massive primary could be responsible for such kind of cyclic period variations. componenthad an off-centered polar spot at the time of their ob- According to his model, the magnetic activity can modify the servations. gravitational quadrupolemoment of the componentsthat induces No other papers based on optical spectroscopy were pub- internal structural variations in the stars, which also influences lished about VW Cephei after the millenium. However, there their orbit. were two publications based on X-ray spectroscopy (Gondoin Many contact binaries also have a detectable tertiary compo- 2004; Huenemoerder et al. 2006), and another one based on UV nent. In fact, Pribulla & Rucinski (2006) summarized that up to spectra (Sanad & Bobrowsky 2014). Gondoin (2004) concluded V=10 mag, 59 8% of these systems are triples on the northern that VW Cephei has an extended corona encompassing the two sky, which becomes± 42 5%, if the objects on the southern sky components and it shows flaring activity. Huenemoerder et al. are also included. These± numbers are based on only the verified (2006) also detected the flaring activity of the system and that detections, but they can increase up to 72 9% and 56 6%, re- the corona is mainly on the polar region of the primary com- spectively, if one takes into account even± the non-verified± cases. ponent. Sanad & Bobrowsky (2014) studied UV emission lines These numbers are just lower limits because of unobserved ob- and found both short- and long-term variations in the strength of jects and the undetectable additional components; therefore, the these lines, which they attributed to the chromospheric activity authors suggested that it is possible that every contact binary of the primary component. is a member of a multiple system. According to some models In this paper, we perform a detailed optical photometric and (e.g. Eggleton & Kiseleva-Eggleton 2001), the tertiary compo- spectroscopic analysis of VW Cep, and examine what kind of nents can play an important role in the formation and evolution changes may have occured in the past two decades. The paper is of these systems. organized as follows: Section 2 contains information about the VW Cephei, one of the most frequently observed contact bi- observations and data reduction process, as well as the steps of nary, was discovered by Schilt (1926). It is a popular target be- the analysis of our photometric and spectroscopic data. In Sec- cause of its brightness (V = 7.30 - 7.84 mag) and its short period tion 3, we discuss the activity of VW Cep based on our modeling ( 6.5 hours), which allows to easily observe a full orbital cycle results, then, in Section 4, we present an O C analysis of the sys- on∼ a single night with a relatively small telescope. The system is tem. Finally, we briefly summarize our work− and its implications worth observing, because it usually shows O’Connell effect and in Section 5. clearly has a third component confirmed by (Hershey 1975); however, there is no perfect model that can explain all the observed properties of the system. 2. Observations and analysis The strange behaviour of the light curve of VW Cephei was 2.1. Photometry discovered during the first dedicated photometric monitoring of the system (Kwee 1966). Several different models have been de- Photometric observations took place on 3 nights in August veloped to explain the asymmetry of maxima e.g. the presence 2014, and 2 nights in April 2016 at Baja Astronomical Obser- of a circumstellar ring (Kwee 1966), a hot spot on the surface vatory of University of Szeged (Hungary) with a 0.5m, f/8.4 RC caused by gas flows (van’t Veer 1973), or cool starspots on the telescope through SDSS g’r’i’ filters. In order to reduce photo- surface (Yamasaki1982). The latter model seemed to be the most metric errors, bias, dark and flat correction images were taken consistent with the observations and dozens of publications pre- on every observing night. The image processing was performed sented a light curve model including starspots. using IRAF1: we carried out image corrections and aperture pho- Instead of the huge amount of photometric data, there is a tometry using the ccdred and apphot packages, respectively. limited number of published spectroscopic observations about The photometric errors are determined by the phot task of IRAF. VW Cep. The first spectroscopic results are from Popper (1948) These errors are in the order of 0.01-0.02 mag, hence they are who published a mass ratio of q = 0.326 0.045. Anderson et al. smaller than the symbols we used in our plots. For differential (1980) took two spectra at two different± orbital phases and de- photometry,we used HD 197750 as a comparison ; although, rived a mass ratio of 0.40 0.05, which was consistent with the it is considered as a variable in the SIMBAD catalog, we could previous value of 0.409 ±0.011 yielded by Binnendijk (1967). not find any significant changes in its light curve compared to Hill (1989) measured the± radial velocities of the components us- the check star, BD+74 880 during our observations. The Φ= 0.0 ing the cross-correlation technique on low-resolution spectra, phase was assigned to the secondary minimum (when the more and determined a new value of q = 0.277 0.007. The next massive primary component is eclipsed), while the secondary spectroscopic study of the system was presented± by Frasca et al. minimum time of the last observing night of both was used (1996) who took a series of low resolution spectra at the Hα- for phase calculation for each season. line; they showed that there is a rotational modulation in the equivalent width of the Hα-profile. Kaszás et al. (1998) obtained the first series of medium resolution spectra of the system. They 2.2. Spectroscopy used the same technique as Hill (1989) to derive the radial veloc- Spectroscopic observations were obtained on 12 and 13 April ities and the mass ratio, but they got a definitely larger value(q 2016 with a R = 20000 échelle spectrograph mounted on the 1m = 0.35 0.01). They also investigated the variation of Hα equiv- RCC telescope at the Piszkésteto˝ Mountain Station of Konkoly alent widths± with the orbital phase and found that the more mas- sive primary component showed emission excess, which they in- 1 Image Reduction and Analysis Facility: http://iraf.noao.edu

Article number, page 2 of 13 T. Mitnyan, A. Bódi, T. Szalai et al.: The contact binary VW Cephei revisited

Observatory, Hungary. The integration time was set to 10 min- Fit utes as a compromise between signal-to-noise ratio and temporal Primary resolution. ThAr spectral lamp images were taken after every Secondary third object spectrum for wavelength calibration. Three spec- Φ = 0.7414 Tertiary tra of an A0V star (*81 UMa) and of a radial velocity stan- dard (HD 114215) were taken on each night for removing tel- luric lines, and for radial velocity measurements, respectively. A total of 69 new spectra of VW Cephei was obtained on these Φ = 0.3424 two consecutive nights with an average S/N of 34. The steps of spectrum reduction and wavelength calibration were done by the corresponding IRAF routines in the ccdred and the echelle packages. We divided all spectra with the previously determined Correlation [arbitrary units] Φ = 0.2642 blaze function of the spectrograph, then we fit all apertures with a low-order Legendre polynomial to correct the small remain- ing differences from the continuum. The obtained spectra were −400 −200 0 200 400 corrected for telluric spectral lines in the following way. We as- Relative velocity [km/s] sumed that the averaged spectrum of a rapidly rotating A0V star contains only a few rotationally broadened spectral lines and the Fig. 1. A sample of cross-correlation functions of different orbital narrow atmospheric lines. We eliminated these broad spectral phases and their fits with the sum of three Gaussian functions. We also lines from the spectrum of the standard star, which resulted in indicated the peaks raised by different components with different lines and colours. a continuum spectrum containing only telluric lines and we di- vided the object spectra with this pure telluric spectrum. The spectra of both VW Cep and the telluric standard star have a of the orbital phase and fitted the velocity points of each com- moderate S/N; therefore, after dividing the object spectra with ponents with the light curves simultaneously using PHOEBE. The the pure telluric spectrum, we get corrected spectra with even radial velocity curve is plotted in Fig. 2. lower S/N values. To help on this issue and carry out a bet- Our newly derived mass ratio of q = 0.302 0.007 is in- ter modelling process, we performed a Gaussian smoothing on side the wide range of the previously published± results, but it all corrected spectra of VW Cep as a final step with the IRAF is slightly lower than the majority of them. Possible reasons of task gauss, which helps filtering out artificial features caused this disagreement are the application of different radial velocity by noise. The FWHM of the Gaussian window was chosen to be measuring techniques, and the different quality of the data. Other 0.7 Å, which was narrow enough to avoid the smearing of the authors, like Popper (1948) and Binnendijk (1967) measured weaker, but real lines in the smoothed spectra. the wavelengths of several spectral lines on low-quality spec- trograms to calculate the radial velocities, while Anderson et al. (1980) derived the broadening functions of the components in 2.3. Radial velocities only two different orbital phases to measure the mass ratio. Hendry & Mochnacki (2000) did not describe their method to Radial velocities of the binary components were determined us- derive the mass ratio, so it’s difficult to compare our results to ing the cross-correlation technique. Each spectrum (before the theirs. There are only two papers in the literature in which the smoothing process) was cross-correlated with the averaged ra- cross-correlation technique is used to derive the radial velocities dial velocity standard spectrum on the 6000 6850 Å wave- of the system. Hill (1989) used low-resolution spectra and his length region excluding the Hα-line and the region− of 6250 − CCFs show only two components, so, he probably measured his 6350 Å. This range we used is indeed narrower than our full velocities on the blended profile of the primary and tertiary com- wavelength coverage. The spectrograph approximately covers ponents, which means that his value of the mass ratio is not reli- the range of 4000 8000 Å, but, because of the necessarily able. Kaszás et al. (1998) applied exactly the same method as we − short exposure times and of the moderate seeing on our obser- did; however, there are differences in the spectral range used for vation nights, the signal to noise ratio is not so high. The S/N the cross-correlation, in the spectral resolution and in the S/N of is decreasing going toward shorter wavelengths which degrades the data: while the average S/N of their spectra is approximately the quality of the CCF profiles, hence we excluded the region 2-3 times higher than that of ours, their spectra have a spectral under 6000 Å. We did the same in the region between 6250 resolution of only R 11000, and we used a seven times wider 6350 Å and above 6850 Å, where the strong telluric lines totally− spectral range to calculate≈ CCF profiles, which is a very impor- cover the features that belong to VW Cep. Nevertheless, we note tant factor in using this method. For checking the limitations of that the wavelength range we used is seven times wider than the the above described method, we applied this technique on the one in Kaszás et al. (1998), who also applied the CCF method to theoretical broadening functions of the system (see in Appendix determine the radial velocities of VW Cep. The obtained CCFs B), which were calculated with the WUMA4 program (Rucinski were fitted with the sum of three Gaussian functions. A sample 1973). of the CCFs along with their fits are plotted in Fig. 1. Finally, heliocentric correction was performed on the derived radial ve- locities to eliminate the effects of the motion of the Earth. The 2.4. Light curve modeling radial velocities are collected in Table C.1. The errors show the The light curves were simultaneously analyzed with the ra- uncertainties of the maxima of the fitted Gaussians. To test the dial velocities using the Wilson-Devinney code based program accuracy of our measurements, we derived the radial velocity PHOEBE2 (Prsa & Zwitter 2005), in order to create a physical of HD 114215. We got -25.35 0.34 km/s, which only slightly model of VW Cephei and to determine its physical parameters. differs from the value found in± the literature (-26.383 0.0345 km/s, Soubiran et al. 2013). We also plotted them in the± function 2 PHysics Of Eclipsing BinariEs: http://phoebe-project.org/1.0/

Article number, page 3 of 13 A&A proofs: manuscript no. Mitnyan_etal_VWCep_rev2

300 Primary component g −1.2 r Secondary component i Fit Fit −1 200

−0.8 100 −0.6 Differential magnitude 0 −0.4 −0.04 −0.02 −100 0

Radial velocity [km/s] 0.02

Residual 0.04 0 0.2 0.4 0.6 0.8 1 −200 Orbital phase Φ = 0.00 Φ = 0.25 −300 0 0.2 0.4 0.6 0.8 1 Orbital phase

Fig. 2. Radial velocity curve of VW Cephei with the fitted PHOEBE model. We note that the formal errors of the single velocity points (see Table C.1) are smaller than their symbols. Φ = 0.50 Φ = 0.75

The surface-averaged effective temperature of the primary com- ponent (Teff,1) and the relative temperatures of the starspots were kept fixed at 5050 K as in Kaszás et al. (1998), and 3500 K as in Hendry & Mochnacki (2000), respectively. We assumed that the orbit is circular and the rotation of the components is syn- Fig. 3. SDSS g’r’i’ light curves of VW Cephei on 8th August 2014 chronous, therefore, the eccentricity was fixed at 0.0, and the along with the fitted PHOEBE models and the corresponding geometrical configuration and spot distribution in different orbital phases. synchronicity parameters were kept at 1.0. For the gravity dark- ening coefficients and bolometric albedos, we adopted the usual PHOEBE = = Table 1. Physical parameters of VW Cep according to our light- values for contact systems: g1 g2 0.32 (Lucy 1967) and A1 and radial velocity curve models compared to the previous results of = A2 = 0.5 (Rucinski 1969), respectively. The following param- Kaszás et al. (1998) (Teff,1, signed with asterisk, was a fixed parameter). eters were varied during the modeling: the effective temperature of the secondary component (Teff,2), the inclination (i), the mass Parameter Thispaper Kaszásetal.(1998) ratio (q), the semi-major axis (a), the gamma velocity (v ), the γ q 0.302 0.007 0.35 0.01 of the three components, and the coordinates and V [km s 1] -12.61± 1.06 -16.4± 1 radii of starspots. The limb darkening coefficients were interpo- γ − a [106 km] 1.412 ±0.01 1.388 ±0.01 lated for every iteration from PHOEBE’s built-in tables using the ± ± logarithmic limb darkening law. i [◦] 62.86 0.04 65.6 0.3 T *[K] 5050± 5050± Light curves obtained on the given nights are plotted in Figs. eff,1 Teff,2 [K] 5342 15 5444 25 3, A.1, A.2, A.3, and A.4 along with their model curves fitted Ω =Ω 2.58272± –± by PHOEBE and the corresponding geometrical configurations in 1 2 m1 [M ] 1.13 1.01 different orbital phases. The residuals are mostly in the range ⊙ m2 [M ] 0.34 0.36 of 0.02 magnitudes, which is comparable to our photometric ⊙ ± R1 [R ] 0.99 – accuracy. One can see that the difference of the two maxima ⊙ R2 [R ] 0.57 – changes slightly from night to night in each filter. This variation ⊙ suggests ongoing surface activity, and it is treated by assuming cooler spots on the surface of the components. The final model sian function applying the FWHM of the transmission function and spot parameters are listed in Tables 1 and 2, respectively. of the spectrograph.A scaled version of this model spectrum (as- suming that the third componentgives the 10% of the total lumi- 2.5. Spectrum synthesis nosity, based on Kaszás et al. 1998) was added to the contact bi- nary model spectrum to mimic the presence of the tertiary com- In order to analyze the chromospheric activity of the system, ponent. We fitted the produced synthetic spectra having different we constructed synthetic spectra for every observed spectra. For temperatures to the observed data (leaving out the Hα-region), spectrum synthesis, we used Robert Kurucz’s ATLAS9 (Kurucz and found that T = 5750 K and log g = 4.0 gives the best-fit 1993) model atmospheres selecting different temperatures (4500 model. This temperature value corresponds to the flux-averaged 6500 K, with 250 K steps). The model spectra were Doppler- mean surface temperature, and it is very similar to that found shifted− and convolved with the broadeningfunctions regarding to by Hendry & Mochnacki (2000) for the unspotted mean surface the observed orbital phases that were computed with the WUMA4 temperature of VW Cep. The best-fit model spectra were then program. As in Kaszás et al. (1998), the tertiary component is subtracted from the observed ones at every . Finally, we also well-resolved in our cross-correlation functions (Fig. 1), measured the equivalent widths of the Hα-profile on the resid- which means that its contribution is not negligible. Accounting ual (observed minus model) spectra using the IRAF/splot task. for this third component, a T = 5000 K, log g = 4.0 model was It is hard to fit the Hα-line of the components with the usual used: it was Doppler-shifted, then convolved with a simple Gaus- Gaussian- or Voigt-profiles on the residual spectra, so we chose

Article number, page 4 of 13 T. Mitnyan, A. Bódi, T. Szalai et al.: The contact binary VW Cephei revisited

Table 2. Spot parameters from the light curve models of single nights.

Parameter 8thAugust2014 9thAugust2014 10thAugust2014 20th April 2016 21st April 2016 Latitude [ ] 46.8 4.0 48.1 3.5 46.7 3.9 46.1 2.6 45.2 1.4 1 ◦ ± ± ± ± ± Longitude [◦] 187.3 0.02 172.2 0.02 179.9 0.01 178.0 0.01 172.8 0.01 1 ± ± ± ± ± Radius1 [◦] 16.0 1.0 18.1 0.9 17.7 1.0 22.4 0.6 23.0 0.4 Latitude [ ] 46.1 ± 3.0 44.2 ± 4.8 45.4 ± 1.5 45.6 ± 6.1 47.0 ± 2.5 2 ◦ ± ± ± ± ± Longitude2 [◦] 342.3 0.02 352.0 0.02 344.5 0.01 343.4 0.05 338.9 0.03 Radius [ ] 19.5 ± 0.4 20.6 ± 2.0 20.4 ± 0.5 21.0 ± 0.7 20.9 ± 0.7 2 ◦ ± ± ± ± ±

Φ = 0.7142 12th April 2016 1.4 13th April 2016

Φ = 0.5047 1.2

Φ = 0.2657 1

Φ = 0.0014 Equivalent width [Å] 0.8 Intensity [arbitrary units]

Observed spectrum 0.6 Synthetic spectrum

6560 6580 6600 6620 6640 6660 6680 6700 0 0.2 0.4 0.6 0.8 1 Wavelength [Å] Orbital phase

Fig. 4. A sample of observed (dashed line) and synthesized (solid line) Fig. 5. The equivalent widths of the Hα emission line profile measured spectra of VW Cephei in different orbital phases. on different nights in the function of orbital phase. For determining the EW values, we subtracted synthetic spectra from the observed ones at every epoch (see the text for details). the direct integration of the equivalent widths. We assumed that the uncertainties of the equivalent width measurements mostly 0.18 come from the error of the continuum normalization, hence they g 0.16 r were determined by the RMS of the relative differences between i the observed and synthetic spectra outside the Hα-region. 0.14

0.12 3. The activity of VW Cep 0.1 0.08 A sample of our observed and synthetic spectra is plotted in Fig. 4, while a more detailed view of the Hα-region along with 0.06 the difference spectra is available in Appendix D (Fig. D.1 and 0.04 D.2). We can see an obvious excess in the Hα line profile, which Differential magnitude strongly suggests ongoing chromospheric activity. The excess 0.02 belongs mainly to the line component of the more massive pri- 0 mary star, while there is no sign of clear emission in the line −0.02 component of the secondary star. It implies that the chromo- 0 0.2 0.4 0.6 0.8 1 sphere of the primary star is way more active, and this is con- Orbital phase sistent with our light curve models where we assumed that spots Fig. 6. Residuals of light curves measured in 2016 compared to spotless are located on the primary. synthetic light curves. In Fig. 5, we plotted the determined equivalent width values as a function of the orbital phase. As it can be seen, the valueson the two consecutiven