We have developed a test for the evolutionary state of W Ursae Majoris binaries by comparing the observed spectral type of 31 of these systems (14 of type W and 17 of type A) with the expected one when their primary component is an unevolved main sequence star. It appears that both the W- and A-type systems have a primary with a mass and radius too large to be compatible with the observed spectral type,so there is no indication that each type should mark a different evolutionary stage.

An attempt is made to explain the variations in the light curve of VW Cep in terms of a starspot model. Based upon Kwee's (1966) observations, an unperturbed light curve without spots is constructed and analysed for the geometrical elements of VW Cep. Then, by assuming that starspots exist on the surface of the primary component, Kwee's individual light curves are further analysed for locations and intensities of the starspots.

The low resolution, phase averaged SWP spectra reveal the trend of increased transition-region line intensities for higher temperatures of ion formation which was previously observed for other active systems. Two series of low resolution LWR spectra covering two full orbital periods have been used to check the photometric results from the ANS satellite and a good agreement is found for the 2200 Å band. Wavelength shifts and changes in the intensity of the blended Mg II emission feature are not inconsistent with the ANS result that the more massive component is the more active one in the W UMa system.

We have investigated the possibility of mass flow due to the heating of the cooler component in a close binary system. The heating may be caused by irradiation from the hotter companion or by other mechanisms such as the spacial coincidence of non-linear “g-mode” oscillations in the cooler star. The 2.4-day period binary U Cep, in which gas streaming has been observed, has been chosen for model calculations. Preliminary results show that such a heating of the lower atmosphere of the cooler star could lead to mass flow at an average rate of 10-9 to 10-7 solar mass per year without the star's necessarily filling its critical Roche surface.

The high resolution ultraviolet spectra of the 4.4-day period binary R Arae, observed in 1980 with the International Ultraviolet Explorer (IUE), show that its continuum flux level varied outside of eclipse by more than a factor of two in ten days, and by over 50 percent within a same orbital cycle. The flux level varied non-monotonically at different wavelengths. The resonance lines of Mg II and Si IV exhibited shortward–shifted absorption components near phase 0.4, indicating the presence of a gas stream toward the observer at a velocity of some –450 to –500 km s–1. Our observations of R Arae with the Einstein satellite show it to be an X–ray source.

We analize the general properties of a group of WUMa-type binaries, which show RSCVn-like activity. The position of these stars in the colour-density and mass-orbital momentum diagrams is studied, but no definite answer can be given on the linkage between short period RSCVn systems (SPG) and WUMa systems with RSCVn-like activity (WWG).

During the four last years (1978–1981), some close binaries of W Ursae Majoris type have been observed from the National Observatory of Athens.

In all cases a two-beam, multi-mode, nebular-stellar photometer has been used, attached to the 48-inch Cassegrain reflector at the Kryonerion Station.

The observed systems are: 44i Boo, VW Cep, AB And, BV Dra, BW Dra, U Peg, V508 Oph. Especially VW Cep and 44i Boo, are very interesting objects; in addition to the light curves that have already been published (Rovithis and Rovithis-Livaniou, 1980–1981), other observations were obtained during the last year and are now at computational stage. The observations of these two systems will be continued during the next years.

In connection with the period change of the AW UMa system, an A-type contact binary, as it was indicated by Woodward (1980) and elaborated by Kurpifiska (1980), we try to analyse with the Wilson-Devinney code all accessible light curves of this star before and after the observed shortening of the period.

Two binary systems UU Lyn (P = 0d.468) and GR Tau (P = 0d.430) are discussed on the basis of BV photoelectric observations and spectroscopic observations which were made between 1980 and 1981 at the Tokyo Astronomical Observatory. The component stars in both systems are found to be still inside their Roche lobes but very close to them.

Attention is directed to the anomalous incidence of W UMa stars, which can be regarded as coming from not only a disproportionately large accumulation among close binary systems with primaries later than around mid-F spectral type, but also as a deficit at early types.

Doubt is placed on the necessity of a straightforward identification of W UMa type light curves with contact binaries; and this allows some reduction in the estimated spatial incidence of contact binaries, from the figure of Van't Veer (1975), to 8 x 10-4 of all stars.

The incidence is considered, with the aid of some simplifying assumptions, as an example of the general evolution of the distribution of binary systems in the primary spectral type - orbital period plane, subject to some known mechanisms of binary evolution.

Some of tile most unique experimental features of the source SS433 are outlined as well as some implications of the theoretical models of this source.

In this paper we present evidence that several of the defining parameters of the Kinematic Model for SS433 are not constant but rather exhibit long term systematic changes. Recent data confirm the existence of the previously reported decrease in the precessional period. The value for this period change, when combined with the observed change in the period of the synodic spectral variations, implies that the orbital period is not significantly changing on a time scale less than 1000 years.

In addition we find mounting evidence for a statistically significant (40) secular change in the cone angle θ at a rate of about -1.5 x 10-3 deg/day. However, t the surprisingly short time scales implied by the observed values of P and θ when combined with estimates of the system age suggest the possible existence of detectable higher time derivatives. This view is supported by the most recent data which suggest a value for P ~ 10-5 (days)-1. It is possible to understand these secular changes in terms of the motions to be expected from an object exhibiting classical precession in response to an external torque.

There are two recognized subsets of cataclysmic binaries that contain white dwarfs: the AM Her stars, whose white dwarf components are sufficiently magnetized to ensure synchronous rotation with the orbital period, and the DQ Her stars, which do not maintain synchronism and probably have been spun up by mass accretion. Both groups of stars, recognized also as X-ray sources, are important as probes of the accretion process, and the radiation mechanisms in the vicinity of the white dwarf. So far, we know five objects belonging to the former group (AM Her, AN UMa, VV Pup, 2A 0311-227, and PG 1550+191), and seven falling into the latter: WZ Sge, AE Aqr, V533 Her, DQ Her, V1223 Sgr, H2252-035, and EX Hya, with the corresponding rotation periods of their white dwarf primaries of 28, 33, 64, 71, 794, 805, and 4020 s, respectively. While the main observational and theoretical efforts have been focussed on the AM Her class, the DQ Her binaries have been investigated to much lesser extent. This is probably because the DQ Her stars have been recognized only very recently as a homogeneous class.

Recent results on mass distributions for white dwarfs and planetary nebulae are presented and compared with current theoretical predictions. Whereas single star evolution leads to final masses predominantely in a narrow interval around 0.6μ0 which can be explained by current mass loss schemes degenerate stars in binaries present a larger range of masses. The average mass of the primaries in cataclysmic binaries seems to be more around 0.7 than 1μ0.

We investigate metal abundances in helium-rich white dwarf atmospheres and check the possibility of variations due to binary evolution.

This communication summarizes the investigations of the relationship between the minimum binary period and the evolution of very short period hydrogen-rich cataclysmic binaries /CB's/. In the first work /Paczyński, 1981/ some ideas were developed concerning an influence of gravitational radiation on the evolution of CB's, and some quantitative predictions were done. In the next paper /Paczyński and Sienkiewicz, 1981, hereafter referred to as PS/ more detailed numerical calculations were performed. We considered a widely accepted picture of such binaries. They are believed to have a white dwarf primary component surrounded with an accretion disk. The secondary component, probably the lower main sequence star or degenerate dwarf, overflows its Roche lobe, and matter flows from the inner Lagrangian point towards the disk. In all known cases but one, the binary orbits are circular. No hydrogen-rich CB's are observed with periods below 80 minutes. Because the nuclear evolution cannot be of any importance for the secondary, we assumed that angular momentum loss due to gravitational radiation is the driving force which causes the mass transfer and a cataclysmic activity. We followed the evolution of such system with a very simple and fast code, and the best input physics actually available /PS, Sienkiewicz, 1981/. We have calculated a large number of evolutionary tracks for many values of a total mass of a binary and the initial mass of the secondary.

The basic binary parameters (masses, system dimensions) of all cataclysmic variables are essentially identical, but there is a great variety in the outburst behaviour. Since there is no evident physical reason for this, it is suggested that nova and dwarf nova variability are periodically repeating states of activity of the same binary. After the nova eruption first follows a postnova state (very low disc mass), later the dwarf nova states BV Pup, U Gem and Z Cam (slowly increasing disc mass and outburst frequency, finally standstills), and finally the UX UMa state (permanent standstill) which is terminated by a new nova outburst. The mass of the disc increases continuously during this cycle. Also the mean mass transfer and accretion rates vary slowly from 1018 g s-1 at beginning and end of the cycle to 1016 g s-1 at mid-cycle, as a U Gem star. The nova outburst is understood in terms of a thermonuclear runaway near the surface of the white dwarf, the dwarf nova outburst is due to intermittent accretion of gas which was accumulated in the outer disc during quiescence (several arguments in favor of this model are given). The time interval between consecutive nova outbursts is of the order of 105 years. The secular evolution of a cataclysmic binary is characterized by a period decrease from 10h to 1h5 in a time scale of 1010 years. The star probably passes the period gaps in the ultra-short period domain in form of no-contact configurations.

The effects of an instantaneous asymmetric supernova explosion in an eccentric binary system are analyzed, taking into account the mass loss out of the system, the influence of the impact of the supernova shell on the companion star and the extra “kick” velocity which a collapsed star might receive in an asymmetric supernova explosion. For a random orientat-ion in space of this asymmetric kick velocity, the survival probability and the runaway velocities are derived and their properties discussed for an explosion occurring at a given position in the initial keplerian orbit and the mean and extreme values of these quantities over one orbit are derived. As an example, the outcome of a possible supernova explosion in the ten best known WR+OB binaries is studied and a comparison is made with the observed run-away OB stars, radio pulsars and binary X-ray pulsars.

Sco X-1 is known to show periods of correlated X-ray and optical activity on a timescale of minutes to days, see e.g. Ilovaisky et al.,1980. The rapid optical variations show typically an amplitude of about 5%. During the 1979 campaign on Sco X-1, blue band observations were obtained on several nights with the ESO 1 m photometric telescope with a time resolution of 2 seconds.

In contrast to the mass spectrum of single white dwarfs which has a single narrow peak at ~0.6 M⊙, the observed mass spectrum of white dwarfs of cataclysmic binaries (CB's) shows a rather uniform distribution of the masses in the range ~0.3 M⊙, to ~1.3 M⊙. The formation of CB's with white dwarfs of less than about 0.8 M⊙ can be understood as the result of a binary evolution according to low mass Case B or Case C with a subsequent spiraling-in in a common envelope. On the other hand the formation of massive white dwarfs of M ≲ 1 M⊙, can be explained as the result of a massive Case B mass transfer yielding a helium star which subsequently undergoes a second Case B mass transfer (so called Case BB evolution). The ultimate product of such an evolution is either a CO-white dwarf with a mass up to the Chandrasekhar limit or a neutron star. The formation of CB's via Case BB evolution requires the binary to undergo at least one, most probably two separate phases of spiraling-in in a common envelope.