This talk summarizes recent work that has been done on cataclysmic variables at Oxford and Cambridge. Full details should be sought elsewhere, either from the authors concerned or from the references given below.

This semi-review will concentrate primarily on geometric and photometric parameters of rotating gaseous disks in cataclysmic binaries. Dimensions, or more precisely - the outer radius of the disk can be estimated from three types of data: (a) From the analysis of eclipses of the hot spot, by assuming that its “radius-vector” is identical with the outer radius of the disk. (b) From the analysis of the eclipses of the disk itself. (c) From the radial velocity data giving the rotational velocity of the disk and the orbital velocity(ies) of the component(s). The resulting radius, corresponding to the peaks of the double emission lines, is not identical but presumably quite close to the outer radius of the disk. From a more detailed analysis of the disk's eclipse one can also obtain an information concerning the surface brightness distribution across the disk.

The International Ultraviolet Explorer was used to obtain spectra at both 7 Å and 0.2 Å resolution of the recurrent nova WZ Sagittae during its 1978 outburst. The first spectra were obtained on 1978 Dec 1.8 UT, approximately 0.7 days after discovery. The decay from outburst was followed until 1979 Jan 1, after which it was too near the Sun to be observed. A post-outburst spectrum was obtained on 1979 July 11. In this paper, preliminary results from the analysis of the low resolution spectra are discussed. These observations will be used to support the similarity between WZ Sge and dwarf novae and to test accretion disk models.

The stars of the AM Herculis group (AM Her, VV Pup, AN UMa and 2A 0311-227) are close binaries containing a mass losing, nondegenerate star and an accreting degenerate dwarf. Their main properties are: the large linear and circular polarization of the optical light, high and low luminosity states, the variable emission line, spectra of H, He and other elements and the identification of these objects with X-ray sources. It is generally accepted that the strong magnetic field of the degenerate dwarf is responsible for these peculiar properties and for the distinction between these objects and the cataclysmic variables (Mitrofanov 1978, 1979a). The polarized optical continuum may be emitted by the accreted magnetized plasma (e.g. Chanmugam and Wagner, 1979), by the magnetized photosphere of the degenerate dwarf (Mitrofanov et al. 1977), or by both sources (Mitrofanov, 1979b). To explain the observed X-rays, Lamb and Masters (1979) showed that a magnetic field about 108 gs is necessary. Unfortunately, the basic prediction of their model - the strong ultraviolet continuum in the spectrum of AM Herculis - appears to be absent (Raymond et al. 1979). Chanmugam and Wagner (1979) proposed a rather different estimate for B of 2 · 108/m*gs (m* = 5 ÷ 25). For further investigations of the AM Herculis-type stars it seems useful to find a direct observational method for measuring the dwarfs′ magnetic fields.

Spectra of AM Herculis taken with IUE show a continuum which is partially eclipsed in phase with the X-ray eclipse and can be separated into two components; a black-body component (kTBB = 25 − 30 eV) which accounts for the ¼ keV X-rays and the eclipsed UV continuum; and a component roughly described by Fν ∂ ν−1 which is not eclipsed.

The quest for optical identification of X-ray sources has revealed a new class of variables currently formed by AM Her, AN UMa, VV Pup, and the most recent member, 2A 0311-227 (for which a variable star designation will be available soon). These objects show many of the variations common to close binary systems where mass transfer is under way. However, they are distinguished by the high degree (10 to 35 %) of linear and circular polarization observed in the optical and near infrared spectral regions. Apparently the polarization is due to cyclotron emission of electrons located in a region where the magnetic field strength is about 108 gauss.

The analysis of the eclipses in the system DQ Her based on the photometric observations of different authors during 1954, 1956 and 1975 allows us to obtain the photometric elements of the system and the structure of disk-like envelope surrounding the white dwarf. The orbit inclination of the system is i = 80° ± 3°, the major axis of the secondary component is 0.40 ± 0.03, the orbit radius is (10 ± 2)1010cm, the masses of the components are: mwd = (0.65 ± 0.35)M⊙, m2 = (0.50 ± 0.15)M⊙. A decrease in the optical luminosity of ≃20% and a total decrease in size of the disk-like envelope of ≃10% is observed from 1954 to 1975. The brightness of the disk-like envelope increases towards its center. The radiation temperature increases toward the center according to the laws T ≃ r−0.20 and T ≃ r−0.35 for 1954 and 1975, respectively. The average value of the radiation temperature of the disk-like envelope is about 104K. The bolometric luminosity of the envelope is (1 to 2)L⊙. The bolometric luminosity of the central white dwarf is anomalously high: ≃(20 to 200 L⊙) and the effective temperature of its surface is ≃(1.0 to 1.7)105K. All these results indicate that the disk-like envelope in the system DQ Her is an accreting disk.

We review and contrast two current theories for the structure of contact binary stars: discontinuity theory and thermal relaxation oscillation theory. We find that the two theories are complementary with the crucial theoretical issue to be resolved being the secular stability of the temperature inversion layer. Critical observational tests remain to be performed.

We extract rotation broadening functions from the spectra of W Ursae Majoris Stars. Using a fast Fourier transform we deconvolve photographic spectra, covering some 500 A including dozens of strong lines, with equivalent spectra from non-rotating stars of similar spectral type. The resulting rotation functions contain information about global features such as the shape of the stellar surface (e.g. mass ratio and degree of contact), gravity brightening and limb darkening. We present preliminary data on the stars VW Cep and ER Vul. The rotation function of the former reveals the presence of the third component found visually by Heintz (1975), while that of the latter shows it to be detached and have mass ratio 0.9.

The structure and evolution of W UMa stars is still unsolved although considerable progress has been achieved in recent years. Most theoretical studies are based on the common convective envelope model by Lucy (1968a,b), which almost satisfactorily explains the nearly equal minima of the light curves. All A-type (Wilson, 1978) and some W-type systems (Whelan et al., 1979) may contain an evolved primary. In this case stable models exist (Hazlehurst, 1970; Moss and Whelan, 1970). Computations performed for the subsequent evolution (Moss, 1971; Hazlehurst and Meyer-Hofmeister, 1973; Rahunen and Vilhu, 1977) show nuclear time scale evolution towards more extreme mass ratio, supplemented by possible thermal time scale oscillations.

After Lucy (1968) introduced the contact-binary model with a common convective envelope, it was envisaged by Hazlehurst & Meyer-Hofmeister (1973) that a sideways flow of convective elements would carry energy from the more luminous star, the primary, to the less luminous star, the secondary, as a result of horizontal pressure variations. Webbink (1977) extended this picture by noting that the interaction between vertical entropy gradients and large-scale smooth circulation currents in the common envelope would provide the necessary redistribution of flux. That is, energy is absorbed by the flow during its vertical motion in the primary and is released during its vertical motion in the secondary. Webbink (1977) mentioned two mechanisms by which a large-scale circulation could be generated: (1) the non-spherically symmetric force field due to rotation and tides which will drive an analogue of classical Eddington-Sweet circulation and (2) differential heating of the base of the common envelope. Although these mechanisms are conceptually different, they are not in practice easy to disentangle, and will certainly both be operating in contact binaries.

We discuss ultraviolet photometry of the eclipsing binary W Ursae Majoris obtained with the Groningen instruments on the Astronomical Netherlands Satellite (ANS). Fifty measurements of this star's brightness were obtained for each of the bandpasses at 2200, 2500, and 3300 Å. This new data is significantly more precise than Rucinski's OAO-2 photometry, enabling us to draw definitive conclusions about the distribution of brightness on the surface of W UMa. The ultraviolet light curves are generally similar in shape to those for optical passbands; the amplitudes are greater in the UV, while the depths of the primary and secondary eclipses are nearly equal at both 2200 and 2500 Å. We have analyzed the (2200 − 3300) color curve to determine how much the color temperature varies over the common envelope, concluding that any such temperature variations must be small. In terms of the traditional surface-brightness parameters for gravity darkening, temperature excess of the smaller component, and bolometric albedo, we find β = 0.03 ± 0.01, X = ΔT/T = 0.000 ± 0.009, and Abol = 0.4 ± 0.4. Further, we conclude that the star is limb darkened considerably more at 2200 than at 3300 Å.

The decreasing period of VW Cep, interpreted on conservative case mass transfer, implies mass flow from primary to secondary. The TRO theory then predicts semi-detached status. The system b-v color is the same at the two minima, to within observational error. This is in marginal disagreement with a hot secondary. It also implies close thermal contact, in disagreement with the postulated semi-detached status.

When there is mass loss from a binary system, the lost mass carries energy and angular momentum out of the system. Therefore, the remaining system must adjust its orbital parameters to the changing values of the total kinematic energy E and the total angular momentum N as the total mass M decreases. The parameters concerned here are : the fractional mass μ, the semi-major axis a, and the eccentricity e.

From a study of the mass ratio function of magnetically active contact binaries it is shown that the great majority of newly formed systems must possess nearly equal components.

We propose that sudden period jumps in contact binaries can be caused by the action of a third companion in a highly eccentric orbit of small periastron distance. Time jumps as observed are easily reproduced for a wide range in mass of the third body but observations may restrict the possible mass range to less than a few times 0.1 Mo.

This is a summary of current observational results of early-type contact binary systems. The classical approaches to derive a photometric solution of a very close binary are found to be incapable of distinguishing whether a system is detached, semidetached, or in contact. A survey of contact systems based on the more modern approach of Wilson and Devinney (1971) and others are summarized in the following. Contact systems with spectral type 0 are: V382 Cyg (Devinney 1976 private communication, Bloomer et al. 1979), V729 Cyg (Leung and Schneider 1978c), UW CMa (Leung and Schneider 1978b), AO Cas (Schneider and Leung 1978); with spectral type B are: SV Cen (Wilson and Starr 1976, Rucinski 1976), V701 Sco (Wilson and Leung 1977, Anderson et al. 1979), BH Cen (Leung and Schneider 1977), RZ Pyx (Devinney 1976 private communication); with spectral type A are: AU Pup (Leung and Schneider 1978a), V1073 Cyg (Leung and Schneider 1978a), V535 Ara (Leung and Schneider 1978a), BV 845 (Leung and Darland 1978), V1010 Oph (Leung and Wilson 1977).

A complete coverage of the spectrum of AG Peg has been obtained between wavelengths 1200 − 7000 A by us, and can be supplemented by IR photometric observations by others. Our IUE observations yield a lower value of E(B-V), about 0.12. The two stellar components are easily recognized, but their characteristics are still rather uncertain.

The cool component may be a normal M1.7 III giant, but the temperature and luminosity of the hot star remain largely indeterminate. Firstly, there are no good models for a hot subdwarf, and secondly, it is difficult to determine the relative contribution of the star itself and the surrounding hydrogen cloud.

The emission lines observed in the UV have double or triple structure, indicating two or three distinct emitting regions.

The preliminary results of a joint study of this symbiotic star with 6 Italian astronomers are reported. The relative intensities of the intercombination emission lines indicate electron densities of 1010 and an upper limit to the dilution factor between 10−4 and 10−6. The resonance emission lines give a maximum thickness in the line of sight. A hot (T between 19000° and 26000°) continuum is interpreted as coming from a hot subdwarf, but the high excitation emission lines appear to be formed near the cool giant component of what is probably a close binary.

This emission-line object was discovered by Kohoutek in 1969. Its light curve was subsequently reconstructed since 1891. Before 1964, the object fluctuated at around 15m, with occasional deep minima as deep as 2.3m. Its spectrum was late M. In 1964 the object brightened up to 14m and then it reached 11.5m in the middle of the year 1966. Since 1966 the object has been slowly decreasing in brightness. Stienon et al. (1974) suggested that the photometric behaviour prior to the outburst could be explained in terms of an eclipsing binary with a period of 960 days. This is now confirmed by the analysis of the post-outburst photometric data and independently by the investigation of the radial velocities of the emission-line component. We have derived the following spectroscopic elements: Period = 950.07d; γ = 37 ± 6 km/s; K1 = 62 ± 8 km/s; f(M) = 23 M⊙; aisin i = 1160 R⊙. We propose a binary model for the symbiotic object consisting of a bright red giant M5 II with a mass of 25 M⊙, and of a very hot compact star of 1 M⊙. Line profiles of the nebular lines as well as Balmer lines suggest the existence of ejected “polar caps” and an equatorial ring around the hot component, much like in novae. It seems that the hot component accretes the material from the giant via strong stellar wind. Changes in the accretion rate can cause the observed optical fluctuations. The major outburst in 1964-1966 was probably caused by a shell flash which also ionized the expanding nebula surrounding the hot component.