Looking for a possible explanation for the debated existence of a correlation between period and eccentricity in binary orbits [of which a diagramme is given in fig. 1.) we made the assumption that the orbital evolution of the binaries could be the consequence of a substantial mass-loss of their components even when these components are late type main sequence stars.

This led first to the consideration of various classes of the areal constant. But it.immediately appeared much better to consider for each of such classes, the total mass of each system instead of its orbital eccentricity and thus to consider the mass-period diagramme rather than the period-eccentricity diagramme. Eleven such diagrammes were considered.

Visual binaries containing a component above the main sequence in combination with a main-sequence star provide an excellent opportunity for determining absolute magnitudes of evolved stars. Some aspects of this technique are discussed, and preliminary results are presented.

About 60% of stars of the disc population in our Galaxy are close binary systems (CBS). Half of the known CBS are spectroscopic binary stars (Kraitcheva et al., 1978).

To know the distribution of a correlation between the masses of CBS components and semiaxes of their orbits is necessary for the investigation of the origin and evolution of CBS. For such statistical investigations, a catalogue of CBS was compiled at the Astronomical Council. The catalogue is based on the 6th Batten catalogue (Batten, 1967), its extensions (Pedoussant and Ginestet, 1971; Pedoussant and Carquillat, 1973) and data published up to the end of 1980 (Popova et al., 1981). Now it is recorded on magnetic tape and contains data on 1041 spectroscopic binaries; 333 of them are stars with two visible spectra. The latter are mostly systems prior to mass exchange and the distribution of physical parameters in these systems reflects the distribution and presumably conditions at the time of formation. Using some assumptions, we can obtain for spectroscopic binaries masses of the components M1 and M2 (or the ratio q = M1/M2) and semiaxes of their orbits. Masses of components with the known sin i were obtained by the usual technique; when sin i was not known, masses were estimated from the spectra. We shall discuss here the distribution of CBS in the M-a plane.

The discovery-and-identification probability for different shaped orbits of spectroscopic binary stars is estimated. The eccentricity distribution observed in the sample of ~1000 binaries with known orbits and appearing as strongly peaked toward e=0 is corrected for observational selection effects. The resulting e-distribution seems to be flat for e in the range ~0.05-0.6 with some excess of circular (or almost circular) orbits and a deficiency of orbits with e ≲ 0.6.

Review and comments on two extremely different samples of binary and multiple stars: systems within 10 parsecs of the sun and systems discovered in galaxies of the Local Group.

Ultraviolet studies, originally undertaken to ascertain the state of the atmosphere of the K-supergiant component of the zeta Aurigae system, have been sidetracked by the discovery of significant accretion effects. An analysis of the phase dependence of the profiles of resonance lines in Mg II and C IV has led to a qualitative model of the wind flow from the K star. At the position of the B star, the flow velocity is about 100 km/sec and the density is 3 x 10-6 cm-3 , leading to a mass loss rate of 2 x 10-8 solar masses per year. This wind interacts with the B star in a shock, which will be described, leading to accretion on the B star at a rate of 4 x 10-10 solar masses per year.

Detached close binary systems define the main sequence band satisfactorily, but very little is known about the masses of giants and supergiants. High-dispersion IUE observations promise an improvement, since blue companions are now frequently found to late-type supergiants. The Interesting cases of μ Sagittarii and in particular of ε Aurlgae are discussed in more detail. The barium star abundance anomaly appears now to be due to mass transfer in interacting systems. The symbiotic stars are another type of binary systems containing late-type giants; several possible models for the hotter star and for the type of Interaction are discussed. The W Serpentis stars appear to be Algols in the rapid phase of mass transfer, but a possible link relating them to the symbiotics is also indicated. Evidence of hot clrcumstellar plasmas has now been found in several ordinary Algols; there may exist a smooth transition between very quiescent Algols and the W Serpentis stars. β Lyrae Is discussed in the light of new spectrophotometry results.

We have inspected the observational data of 114 Algol-type binaries in order to clarify some general aspects of their evolutionary scenario. Through a comparison with the corresponding available statistical analyses of large sample of eclipsing and spectroscopic binaries, we have found a larger than normal concentration of low values of masses, binary separations and specific orbital angular momenta for Algol-type stars. Furthemore, high mass ratios predominate in massive and early-type Algol systems, whereas high total masses are generally accompanied by less prominent oversized and overluminous properties of the mass-losing components.

We have examined the observational data of 102 Algols in order to clarify the implications on their evolutionary scenario of various assumptions concerning mass and angular momentum loss during mass transfer. We have found that case B mass exchange is strongly favoured for Algols of relatively low total mass (~ M < 7 Mʘ), while case A predominates, though not so widely as expected in Algols of higher total mass.

Ultraviolet light curves have been obtained for the quiescent dwarf novae U Gem and VW Hyi. The amplitude of the hump associated with the accretion hot spot is much smaller in the UV than in the visible. This implies that the bright spot temperature is roughly 12000 K if it is optically thick. A hotter spot would have to be optically thin in the near UV. The flux distribution of U Gem in quiescence cannot be fitted by model spectra of steady state, viscous accretion disks. The absolute luminosity, the flux distribution, and the far UV spectrum suggest that the primary star is visible in the far UV. The optical-UV flux distribution of VW Hyi could be matched roughly by our model accretion disks, but the fitting is poorly constrained due to the uncertainty in its distance.

Statistical models for binaries as a vhole were selected; then statistical properties of close binaries -the proportions of spectroscopic and eclipsing binaries and the distributions of the K velocities and of the depths of eclipse- were computed for each model.

Optical photometric and spectroscopic observations of the eclipsing binary SV Cen were used to derive orbital elements and absolute dimensions of the peculiar early-type contact system. UV spectroscopic observations obtained with the IUE satellite yielded the detection of mass loss from the system forming an expanding circumbinary envelope. SV Cen exhibits a non-uniform rate of period decrease, with major variations on a time scale of a few years. The period changes are explained in terms of loss of angular momentum carried away by the ejected matter. Short-term fluctuations of the stellar wind on a time scale of a few hours were found from an analysis of the envelope line profiles of UV resonance lines. A model of the system is derived, and the evolutionary stage and interaction processes are discussed.

A numerical method for 3D magnetohydrodynamical investigations of accretion disks in close binary systems is presented, which allows for good spatial resolution of structures (hot spot, accretion column). The gas is treated as individual gas cells (pseudo-particles) whose motion is calculated within a grid consisting of one spherical inner part for 3D MHD and two plane outer parts. Viscous interactions of the gas cells are taken into account by a special treatment connected with the grid geometry.

We present one result of 2D hydrodynamical calculations for a binary applying the following parameters which are representative for Cataclysmic Variables: M1 = 1 Mʘ, r1 = 10-2 Rʘ, M2 = 0.5 Mʘ, p = 0.2 d, M = 10-9 Mʘ y-1.

Column density and radiative flux distributions over the disk are shown and briefly discussed by comparison with the theoretical understanding of these Dwarf Novae drawn from observations.

Quasi-simultaneous photometric and polarimetric observations of T Tau and RY Tau show that the proper polarization of these stars barises in the visual region in their gaseous emitting envelopes. The polarization of T Tau shows cyclic variations with a period of 5h18. That of RY has two different modes of variation at brightnesses V > 11m and V < 11m; it is possibly connected with the suspected binary nature of this star.

Basic information on empirical determinables for the system S Cnc are reviewed. Whilst the photometric information can be clearly analysed along well known lines, the spectrographic information is far from clear and requires further study, though a semi-detached configuration, at least, can be confidently deduced.

The potentially important question of some physical relationship of the variable to the open cluster Praesepe is considered, but the result is in the negative.

The evolutionary status of the binary is examined in a general way by making use of some summary formulae, some guidelines for which were taken from previous more general work of Refsdal and Weigert. S Cnc is in good overall agreement with the low mass Case B theoretical mode of binary evolution, and even the absence of a detectable rate of period variation is shown to be not in serious conflict with this picture. Interesting close comparisons with the well known example AS Eri are possible.

Equilibrium structural models are computed for a thick, selfgravitating disk in a binary system. Accretion onto the star is limited by the star's rapid rotation (the system is a double-contact binary). The potential formulation is taken from a previous paper, and represents the gravitational potential as that of a massive wire. Corrections to the stellar structure differential equations for the distorted geometry are applied, and the equations are integrated and solved by the fitting point method. The energy is supplied by viscosity. Energy transfer is by convection, and is appreciably superadiabatic throughout the disk. A mass of 0.5 Mʘ is assumed. Representative results are: “central” temperature, 67000 K; “central” pressure, 5 x 1011 dynes/cm2; “equal volume” radius, 17 Rʘ; luminosity, 5 x 103 Lʘ. The model “radius” is in excellent agreement with the observational value for β Lyrae. The model luminosity is slightly higher than the available rate of expenditure of gravitational energy, indicating that a lower disk mass (perhaps 0.25 Mʘ) should be tried.

The structure and evolution of contact binaries is an extremely difficult problem in theoretical astrophysics.Several attempts have been made to set up a satisfactory model, see e.g. Lucy,1976, Flannery, 1976, Shu, Lubow and Anderson, 1976, Robertson and Eggleton, 1977, Rahunen and Vilhu, 1977 and others. There are, roughly, two lines of sight: Shu, Lubow and Anderson postulate the existence of a contact discontinuity in one of the two components of a contact binary (DSC Theory); in this case age zero models in thermal equilibrium are possible. The other authors find that unevolved contact binaries cannot achieve thermal equilibrium; therefore, relaxation oscillations on a thermal time scale are expected (TRO Theory). For a detailed discussion of the different effects see Lucy and Wilson,1979.

We give a great number of arguments for the hypothesis that several epochs of star formation have taken place at different times in the old galactic cluster NGC 188.

From the last burst of star formation in this cluster, not more than a few times 108 years ago, 4 contact binaries are still now visible as W UMa stars. With the aid of a simplified probability calculation we argue that these 4 contact binaries are physically related and that the alignment of the orbital axes is not accidental.

Using angular momentum loss estimates from single star studies, it is shown that detached binaries are good candidates as progenitors of contact binaries. Three theories constructed for contact binary evolution (DSC, TRO and AML, see Fig. 1) are discussed. The DSC and TRO theories require a contact binary formation mechanism which produces unequal components (fission) while the AML theory can start from equal components (initially detached binaries). In all theories the end-product is a single star.

The stability of unequal entropy models was studied using a formula (2) which couples the energy transfer with the depth of contact and with the entropy difference. The models experience cyclic behaviour on a time scale of 106-107 years (Fig. 2) and the contact never breaks even on the nuclear time scale. This is the important consequence of a formula of type (2). Similar behaviour (with similar formula) is expected also for TRO models and even for DSC models if the discontinuity can be preserved during one cycle period.

The DSC and TRO theories, which at first sight look quite different, are in fact complementary. The most probable contact binary theory is perhaps a suitable combination of all three. In this theory angular momentum loss is the new and important factor which may manifest itself in the UV- and X-ray activity observed in W UMa stars.