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J. galaxies, Carnegie Institution of Washing­ de Vaucouleurs, G., 1987, lAU Symp. 127, 278, l1. ton Publication 635. p.3. Nieto, J.-L. and lelievre, G., 1982, Astron. Schneider, D. P., Gunn, J. E. and Hoessel, Westerlund, B. E. and Smith, L. F., 1964, Aus­ Astrophys. 109,95. J. G., 1983, Astrophys. J. 264, 337. tra/ian J. Phys.. 17, 340. Phillips, M. M., 1979, Astrophys. J. 227, Schweizer, F., 1980, Astrophys. J. 237, 303. l121. Long-Term Walraven Photometry of Cataclysmic Variables s. VAN AMERONGEN and J. VAN PARAOIJS, Astronomicallnstitute "Anton Pannekoek", University ofAmsterdam, the Netherlands Ouring the last seven years we have lerian orbit around the white dwarf. It is in a spasmodic way. Long time intervalls been observing cataclysmic variables generally believed that viscous pro­ of the order of months of a low accretion (CV's) with the Walraven photometer cesses give rise to exchange of angular rate (and therefore luminosity) are in­ behind the gO-cm Outch telescope at La momentum, by wh ich an initially formed terrupted by outbursts, lasting for a few Silla. An advantage of this photometrie ring spreads and forms a flat disk-like days to about two weeks, during which system for strongly variable blue sour­ configuration, the accretion disk, the accretion rate increases by a large ces is that it allows simultaneous detec­ around the white dwarf. Matter trans­ factor (see Fig. 2). tion in five passbands, extending from ferred from the secundary gradually spi­ the optical (5400 to the ultraviolet rals inward along quasi-Keplerian orbits. A) Intermediate Polars (3200 A). We have observed these CV's Ouring this diffusion of the particles, the by monitoring them for several hours, other half is radiated away. In these cataclysmic variables the and reduced the measurements diffe­ This radiation from the accretion disk white dwarf has a magnetic field that is rentially with respect to nearby compari­ generally dominates the optical and UV strong enough to dominate the motion son stars. luminosity of cataclysmic variables. The of the inflowing matter within a certain Results obtained so far include mea­ kinetic energy of the inflowing material is distance from its centre. Within this surements of changes in the rotation eventually dissipated close to the white "magnetospheric radius" the matter is periods of accreting white dwarfs in dwarf surface, e. g. in a transition region channeled onto regions near the CV's, quite strong upper limits to secu­ from the accretion disk to the white magnetic poles of the white dwarf. In­ lar changes in optical brightness of dwarf surface. side this magnetosphere an accuration dwarf novae (ON) between outbursts, Based on observational characteris­ disk cannot exist; what fraction of the and the recognition that accretion in­ tics, cataclysmic variables have been accretion disk remains depends on the stabilities similar to ON outbursts also divided into several groups. Ouring our size of the magnetosphere, relative to occur in systems not classified as such. observing project, we have paid most of that of the Roche lobe around the white our attention to the following types of dwarf. cataclysmic variables: Introduction Brightness Variations Cataclysmic variables are close bi­ DwarfNovae nary stars consisting of a white dwarf Most cataclysmic variables with an and a low-mass « 1 MG) near-main­ In the dwarf-nova systems the accre­ accretion disk show brightness varia­ sequence companion. This "secondary" tion rate onto the white dwarf changes tions with the orbital period, wh ich are star fills its so-called Roche lobe, i. e. the critical equipotential surface, outside which matter is no longer bound to the star (see Fig. 1). At the point on the line through the two stellar centres, where the Roche lobes of the secondary and the white dwarf touch each other, mat­ ter from the secondary can easily fall into the gravitational potential weil of the white dwarf. Such matter, flowing from the secon­ dary star, has angular momentum with respect to the white dwarf, due to the orbital revolution of the binary. It will Figure 1: A sketch of the structure ofa cataclysmic binary. Matter from the secondary falls into therefore settle into a more or less Kep- the potential weil of the white dwarf and hits the accretion disk at the bright 01' "hot" spot. 44 3 ~--r----------.------------r-,-"-~ 3 novae, the SU UMa systems, that show '. two rather different types of outbursts 2 2 (superoutbursts and "normal" out­ bursts). Superoutbursts are different ! . from normal outbursts - in their rate of lL i I· 11 il I I I; --"- ---'- .1....- .-1 oeeurrenee (Iess often), their duration 2445900 2445950 (Ionger), and their maximum luminosity 2446000 (higher). Apart from this, it is found that Figure 2: An overview of the optical observations of the SU UMa-type dwarf nova VW Hyi during superoutburst, the optieal bright­ during July-November 1984, when the source went into a normal outburst 4 times and finally ness is strongly modulated; these so­ went into a superoutburst. Drawn is the Walraven-B-flux in log F", with F,. in units of milli­ Jansky's, against Heliocentric Julian Day. ealled superhumps reeur with aperiod that is slightly longer than the orbital period; also this superhump period is due to a so-ealled "bright spot" at the through the disk to the white dwarf. not exaetly eonstant, but inereases outer edge of the disk, where it is hit by During quieseenee, when the aeeretion through the superoutburst. The the stream of matter flowing in from the rate onto the white dwarf equals the superhumps have always been a big seeondary (see Fig. 1). In systems with a mass transfer rate from the seeondary, problem, whieh resisted theoretieians' magnetized white dwarf the light ean no seeular ehanges are expeeted in the attempts to model them. However, re­ also be modulated with the white dwarf aeeretion disk's state. eently Whitehurst has proposed that the rotation period, due to the higher One of the aims of our long-term superhump phenomenon might be the emissivity from the magnetie pole re­ photometrie projeet of eataelysmie var­ result of a mass-transfer burst from the gions and their general non-alignment iables was an attempt to distinguish be­ seeondary, leading to a slowly preeess­ with the rotation axis of the star. In these tween these two types of model, by ing, more or less eeeentrie disk, as the systems the light ean also be modulated studying the possible seeular brightness result of tidal distortion by the seeon­ with a third period, the beat period be­ variations of dwarf novae in quieseenee. dary of the orbits of partieles in the outer tween the white dwarf rotation period For this purpose we have observed the disk regions [4]. His model sueeessfuily and the binary revolution period, whieh system VW Hyi during 42 nights over a aeeounts for many of the strange prop­ is the rotation period of the white dwarf time interval of 4 months, during whieh 5 erties of the superhumps. in the referenee frame of the binary. outbursts oeeured. These observations Our Walraven observations of the Modulations with the beat period are were eoordinated with observations superoutburst of VW Hyi, wh ich were due to proeessing of eleetromagnetie with EXOSAT, Voyager, IUE, and Walra­ done in five ehannels simultaneously, radiation from the rotating white dwarf ven [3] (see Fig. 2). The main outeome of have led to the diseovery of another by parts of the binary system that this study is that there is no evidenee for strange property of superhumps: the are not distributed axially-symmetrie a signifieant trend in the optieal bright­ whole superhump-modulation light around the white dwarf, e. g. the seeon­ ness from the system between out­ eurve arrives later when seen at longer dary and the bright spot on the aeere­ bursts (see Fig. 4). The same was found wavelengths. The total shift from 3200 A tion disk's outer rim (see Fig. 3 for an in the X-ray and UV observations (if any to 5400 A is + 0.06 in phase of the example of these modulations). change is diseernable it is a seeular superhump period [5]. We eannot offer deerease, not an inerease). This result is an explanation for this finding. not easy to reeoneile with simple ver­ DwarfNovae sions of the disk-instability model. Intermediate Polars At the end of our eampaign VW Hyi Two types of models have been pro­ went into a so-ealled superoutburst. VW In intermediate polars the influenee of posed to explain the outbursts of dwarf Hyi belongs to a subgroup of dwarf the white-dwarf magnetie field does not novae. In the disk-instability model [1] the aeeretion disk ean exist in two diffe­ rent modes, eorresponding to a high and a low value of the mass aeeretion 1 .