Cool stars show evidence of dynamo activity which is stronger with more rapid rotation. Tidal friction in a moderately close binary can be a cause of relatively rapid rotation, so that cool components in such binaries are presumably liable to stronger stellar winds than single cool stars. As a consequence, the binary can be subject to orbital angular momentum loss. Both the mass loss and the orbital angular momentum loss can be on a timescale comparable to nuclear evolution in a red subgiant, or even faster. RS CVn stars probably give the best possibility of measuring these processes, although some observational data are difficult to reconcile with simple theories.

Barium stars, and symbiotics, may both be affected by these processes. They must be the products of evolution of moderately wide binaries, as must such objects as cataclysmic variables. I attempt to define the ranges of zero-age parameters necessary to produce such varied objects. A simplistic model of the distribution of stars brighter than 6th magnitude (a ‘Theoretical Bright Star Catalogue’) suggests that for every three Ba stars with a measurable orbit, there should be one main sequence ‘Ba star’.

The history of the first detection of an accretion disk in a interacting binary system is presented. The nature of transient and permanent disks in Algol-type binaries is discussed as are the problems associated with the interpretation of emission line profiles from the disks in cataclysmic variable stars. The new technique of eclipse mapping from continuum photometry is reviewed. Some results of Doppler tomography of emission lines are presented.

The problem of the stability of wind accretion onto compact objects is examined. Recent analytical and numerical calculations show that in two dimensions, Bondi-Hoyle accretion flows are unstable to a “flip-flop” instability. The instability can manifest itself as bursts in the accretion rate and as a random walk-type spin-up, spin-down behaviour of the accreting compact object. The nature of the flow in three dimensions needs further clarification. Possible observational implications are reviewed.

I give a brief discussion of accretion disc instabilities, concentrating mainly on tidal instabilities caused by the presence of a binary companion. The superhumps observed in superoutbursts of SU UMa dwarf novae probably result from the excitation of a resonance in the accretion disc near the 3:1 commensurability with the binary orbit. This resonance can only appear for mass ratios q = M 2/M 1 < q erit ≃ 0.25 – 0.33: for larger mass ratios the available resonances are considerably weaker. Application of this picture to other types of binary suggests that the condition q < q erit may be necessary but not sufficient. Further, some cataclysmic systems show phenomena which could be tidal in origin even though the condition evidently fails.

Accretion disk thermal instability models have been successful in accounting for the basic observations of dwarf novae and the steady behavior of nova-like systems. Models for the dwarf-nova like variability of the old nova and intermediate polar GK Per give good agreement with the burst amplitude, profile and recurrence time in the optical and UV. A month-long “precursor plateau” in the UV is predicted for the expected 1992 outburst prior to the rise to maximum in the optical and UV. The models for the time scales of the outbursts and corresponding UV spectra at maximum are consistent with the inner edge of the accretion disk being essentially constant between quiescence and outburst and a factor of four larger than the corotation radius. These conclusions represent a challenge to the standard theory of magnetic accretion. Disk instability models have also given a good representation of the soft X-ray and optical outbursts of the X-ray novae A0620-00 and GS2000+25. Formation of coronae above the disk, heated by magneto-acoustic flux from the disk, may account for the temporal and spectral properties of the hard X-ray and gamma ray emission of related sources such as Cyg X-1, GS 2023+33 (V404 Cyg), 1E 1740.7-2942 (the “Galactic Center” Einstein Source), and GS 1124-683 (Nova Muscae).

Three distinctly different types of outburst phenomenon in X-ray binaries are discussed: (1) the so-called type 1 burst which is a thermonuclear flash of matter accreted on the surface of neutron stars, (2) the so-called type 2 burst which is a burst-like X-ray emission caused by a spasmodic accretion, and (3) the nova-like outburst which is triggered by a sudden commencement of accretion.

Spectral calculations show that a model based on the thermonuclear explosion of a degenerate carbon/oxygen white dwarf provides excellent agreement with observations of Type Ia supernovae. Identification of suitable evolutionary progenitors remains a severe problem. General problems with estimation of supernova rates are outlined and the origin of Type Ia supernovae from double degenerate systems are discussed in the context of new rates of explosion per H band luminosity, the lack of observed candidates, and the likely presence of H in the vicinity of some SN Ia events. Re-examination of the problems of triggering Type Ia by accretion of hydrogen from a companion shows that there may be an avenue involving cataclysmic variables, especially if extreme hibernation occurs. Novae may channel accreting white dwarfs to a unique locus in accretion rate/mass space. Systems that undergo secular evolution to higher mass transfer rates could lead to just the conditions necessary for a Type Ia explosion. Tests involving fluorescence or absorption in a surrounding circumstellar medium and the detection of hydrogen stripped from a companion, which should appear at low velocity inside the white dwarf ejecta, are suggested. Possible observational confirmation of the former is described.

It is often assumed that a binary begins to interact when one of its components makes contact with its Roche lobe, thus “switching on” a new evolutionary process. The example of Y Cygni is used to illustrate the view that the whole lifetime of a binary helps to determine whether or not its components will interact. Of particular importance is the interval between the formation of a binary and the arrival of its components on the main sequence, during which probably all binaries are interacting. Barring accidents, the properties of the components when they reach the main sequence will define the whole subsequent history of the system, including whether or not there will be subsequent phases of interaction triggered by contact with the Roche lobe. Like any other mechanical system a binary will tend towards the state of lowest energy consistent with the constraints on it. This it can do by losing mass, equalizing the component masses, or reducing its separation. We therefore expect systems to tend to small masses to mass-ratios of unity, or to coalesce into single stars. In any given system, probably all three tendencies exist, but one dominates. For example, W Ursae Majoris systems may be fusing into single stars. The rotation, chemical composition, and magnetic fields of the component stars may modify the evolution of a binary and be responsible for the variety of interacting systems that we observe. Most interacting pairs are losing mass to the interstellar medium, so a complete study of binary evolution must consider not only the dynamical, but also the chemical, effects of binary systems on the evolution of the Galaxy.

A review of IAU Symposium 151 on Interacting Binary Stars is given along with discussions of the present and future prospects of research in close binary stars.

We have studied a sample of selected eclipsing binaries with accurate absolute dimensions in order to check the validity of available theories of tidal evolution in terms of synchronization and circularization time scales. The use of integrated values, using theoretical evolutionary tracks to compute the change with time of radius, luminosity and internal structure, provides a powerful tool for the prediction of observable parameters. Pseudo-synchronization at periastron is the best approximation to stellar rotation in eccentric systems. While most systems are found to show synchronized velocities, the presence of eccentric orbits shows that circularization process are still active during the main sequence stage.

High resolution spectrum of the VV Cephei type star δ Sge (M2 II + A0 V) observed with the IUE satellite between 1979–1985 have been analysed. The radial velocity curves of non blended absorption features have been formed. Observed radial velocities are compared with the theoretical radial velocity curve due to orbital motion. We found that observed radial velocity variation is the resultant of radial velocity of hotter component and wind velocity around this star. The wind is perturbed by hotter component. Matching the UV continuum fluxes obtained from IUE high resolution spectra to fluxes of Kurucz model atmospheres yields Te=11000±500 °K. Thus the spectral type of hot component was found as A0.

A selected sample of eclipsing binaries with well-defined apsidal motion rates and accurate absolute dimensions has been analysed and compared with most recent evolutionary models of the stellar internal structure. A reasonably good agreement has been achieved when models including convective-core overshooting, mass loss, and stellar rotation in the evolutionary code, are used. The systematic difference in log k2 reported by previous authors between observed and theoretically predicted values is no longer detected for stars with well-determined absolute dimensions, and a few anomalous cases can be easily explained in terms of the validity of the adopted assumptions.

Absolute dimensions of the close binary RX Her are derived based on a new set of photometric elements obtained with a modified version of EBOP code. The observed properties of both components are in good agreement with theoretical evolutionary models. However the predicted apsidal motion rate is not concordant with the observational evidences.

We present the first results of a continuous monitoring of Algol with IUE along 1.5 orbital period (4 1/3 days). A total of 32 high resolution spectra were obtained. Algol represents the final slow stage of mass transfer. The spectra show the presence of high ionization lines (NV, SiIV, CIV) due to the heating of the gas that impacts the accreting star. The depth of the primary eclipse is larger at shorter wavelengths and the shape of the continuum out of the eclipse is compatible with a B star, ruling out the presence of an extra light source at these wavelengths.

The effect of tidal perturbation to stellar pulsation is a relatively underdeveloped problem in the theory of variable stars. We derive amplitude equations describing the resonances between pulsational modes and orbital motion taking into consideration the rotation of stars as well. In the case of δ Scuti stars the two-mode-tidal resonance was found to be the most powerful effect. If the difference between frequencies of excited and damped mode is close to the orbital frequency, parametric excitation of the damped mode may occur, while the other mode loses energy. We discuss this effect for a wide range of parameters.

Data on a selected sample of semidetached Algol-type eclipsing binaries with well-known absolute dimensions have been collected. The strict criteria of selection adopted are given together with the provisional form of the catalogue. This includes besides the absolute dimensions other relevant parameters that may be of interest.

A set of evolutionary sequences for the two components of massive close binaries in the mass range 9 to 40 M⊙ for mass ratios 0.9 and 0.6 has been calculated. The used code accounts for mass transfer according to case B, i.e. mass exchange after core hydrogen exhaustion, during semi-detached and contact phases. All models were computed for a single initial chemical composition X = 0.7, Z = 0.02. Stellar wind mass losses were taken into account, as well as convective core overshooting (α = 0.25). The Roche lobe overflow was calculated according to a simplified hydrodynamical treatment. For the mixing of infalling material with the originally present layers the process of thermohaline mixing was adopted. It was assumed that 50% of the matter lost by the primary is accreted by the secondary.

Wide and narrow Hα lightcurves of R CMa were analysed using Wilson-Devinney (WD) and Information Limit Optimisation Technique (ILOT) approaches. A range of mass ratios, tested by both methods, led to an optimal estimate of around 0.45, at variance with the spectroscopic value. The distortion on the light curve affects the modelling, and so, in a second fitting, this was represented by a ‘hot spot’, associated with mass transfer effects. A semi-detached configuration was then derived. This is supported by the form of the Hα index variation, which has also been modelled. Although thus appearing as a ‘classical Algol’ system, R CMa retains its inherent peculiarity of low mass ratio with low period, which cannot be reconciled with conservative evolution scenarios.

Considering a small number of representative systems among them U Cephei, SV Centauri, V471 Tauri, EM Cygni, the case is made for irregular period variations playing a substantial role in the observed changes of close binary periods.