Recent observational studies of the properties of binary systems among young stars indicate that the majority of binaries are formed very early in the history of a star, perhaps during the protostellar collapse. Major observational facts to be explained include the overall binary frequency, the non-negligible occurrence of multiple systems, and the distributions of period, eccentricity, and mass ratio among the individual binaries. Theoretical calculations of the collapse of rotating protostars during the isothermal phase indicate instability to fragmentation into multiple systems. This process in general produces systems with periods greater than a few hundred years, although somewhat shorter periods are possible. Fragmentation during later, optically thick, phases of collapse tends to be suppressed by pressure effects. Therefore, major theoretical problems remain concerning the origin of close binaries. Fission of rapidly rotating stars, tidal capture, and three-body capture have been shown to be improbable mechanisms for formation of close binaries. Mechanisms currently under study include gravitational instabilities in disks, orbital interactions and disk-induced captures in fragmented multiple systems, hierarchical fragmentation, and orbital decay of long-period systems. Single stars, on the other hand, could result by escape from multiple systems or by the collapse of clouds of low angular momentum, coupled with angular momentum transport after disk formation.

The interaction of disks and young binary stars is discussed, with particular emphasis on disk structure, the existence of accretion at the stellar surfaces and binary orbital evolution.

Our knowledge of the evolutionary processes taking place in interacting binary stars is based primarily in our understanding of the stellar structure and evolution of single stars. The determination of absolute dimensions in close doble-lined eclipsing binaries, with well-detached components, is the best empirical approach available into stellar structure.

Important physical parameters and processes can be studied in some detail within the framework of detached main-sequence binaries, e.g., initial chemical composition, the mixing-length ratio, the relevance of convective overshooting, interactions between matter and radiation in the stellar interior, the concentration of mass towards the center of the stars or the response of stellar envelopes to tidal influences by an external gravitational potential.

A combination of observational results from photometry and spectroscopy are used to obtain relevant stellar parameters: absolute dimensions, ages, chemical composition, apsidal motion rate, degree of synchronization and circularization, etc., and a comparison with theoretical models allow us to derive some information about the above mentioned physical mechanisms.

We investigate the different aspects that govern the interaction of post-main-sequence evolution of binaries, using a new, homogeneous set of computations. The set describes the evolution of both components through the phase of mass transfer, till the end of the core helium burning of the primary. The mass range is 9 to 40 Mo, the mass ratios are 0.9 and 0.6 (and introducing 0.99 as a newcomer). Both for small and large masses we discuss the consequences of non-conservative mass transfer. Using a simple concept for the fraction β of transferred matter, we look to its value at the onset of mass transfer for various mass ratios and periods. Exploration of the phase space of interacting binaries reveals the influence of the various parameters on the dimensions of the resulting systems after mass transfer. Special attention is given to binaries with mass ratios very close to one. Their secondaries evolve directly into yellow supergiants such as observed in the LMC.

Recent progress in the study of ζ Aur and VV Cep systems is reviewed, with an emphasis on what has been learned about the extended atmospheres and winds around the cooler components.

The origin and evolution of contact (W UMa) binaries are discussed with the help of the braking law of eq.(1). Best observationally justified parameter-values in (1) for W UMa, CV and LMXB binaries are: α ≈ 1.5, γ = 2 and f computed from eq.(2). A semi-empirical relation (eq.(5)) between the scaled magnetic torque and the fractional X-ray luminosity is proposed. First attempts of simultaneous photometric and spectroscopic surface imaging of contact binaries are described.

The picture emerging from recent studies of RS CVn-type systems indicates the presence of very active stars showing typical solar-like activity phenomena, such as spots and flares, and possibly mutual interactions. The binary nature of RS CVns is certainly important in enforcing high stellar rotation rates, but the actual clue to the understanding of the intrinsic variability of the component stars resides in their internal structure, where the appropriate physical conditions are met for the generation and intensification of strong magnetic fields, as prescribed by the αω-dynamo models. The most significant results have been derived from multi-wavelength, coordinated observations and long-term monitoring programs.

Recent highlights include: a) the mapping of compact atmospheric structures at various temperature regimes by light curve modelling and spectral (Doppler) imaging techniques; b) clear evidence of long-term activity cycles on RS CVn and other types of interacting binaries; c) the detection and measurement of surface magnetic fields, as derived from the differential Zeeman splitting of spectral lines.

These results clearly demonstrate that the study of RS CVn stars can play a very fundamental role in the understanding of basic stellar physics, as well as in interpreting the characteristic variability of interacting binaries.

Accretion disks in cataclysmic binaries are optically thick (in the vertical direction), except for the case of inner parts of cool disks in dwarf novae at quiescence, which are optically thin. In long period Algols disks are cool and optically thick; their atmospheres can be quite extensive.

Except for inclinations close to i = 0°, disks obscure either one or both hemispheres of the central star, the obscuration being either partial or total. In particular:

1. (a) In stationary accretion cataclysmic binaries (old novae and nova-like systems), at inclinations close to 90° the white dwarf is fully obscured; at i < 85° only its top hemisphere becomes visible.

2. (b) In dwarf novae at quiescence, at inclinations close to 90°, the obscuration of the white dwarf is either complete (for disk radii Rd ≥ 4 × 1010 cm), or limited to its equatorial parts (for Rd ≤ 2 × 1010 cm); at i ≈ 85 – 88° the top hemisphere becomes fully visible, while the bottom hemisphere is fully obscured; at i < 85°, due to the low optical thickness of the inner disk, parts of the bottom hemisphere near the equator also become visible.

3. (c) In long period Algols the situations is similar as under (a); in addition, depending on inclination, part of the obscuration can be due to the atmosphere of the disk.

Neutron stars and a few black holes in binaries reveal their presence by emitting X-rays when they accrete gas from their companions via a wind or disk. Related objects include SS 433, Geminga, gamma ray bursters, TeV/PeV sources, and the source in CTB 108. Systems with secondaries 8 Mo are the natural descendents of main sequence OB binaries. Those with secondaries ≤ 1 Mo arguably form some other way. These systems display a wealth of structure in both wavelength and time domains, much of which is reasonably well understood. Among the things we would like to know more about are the masses and rotation periods of the neutron stars in the two main kinds of systems.

The Barium stars are reviewed in terms of their known binary nature and their quantitative abundance peculiarities. It is concluded that the most likely explanation of the 12C and s-process overabundances observed in the Barium stars is mass transfer from a companion, which is now a white dwarf, but was once a 4He-burning thermally-pulsing asymptotic giant branch star.

Binary pulsars and others with weak fields and rapid rotation now number several dozen and appear to have been spun up by close binary mass transfer. Some are lineal descendents of X-ray binaries; others may result from accretion-induced collapse of binary white dwarfs or from captures, exchanges, and collisions in clusters. Seven accurate neutron star masses, 1.27-1.44 Mo, have been measured so far. Outstanding issues include the dominant formation process in various sites and the time history of the neutron stars' magnetic fields in and out of binaries.

Many new results have been obtained up to now in the problem of Wolf-Rayet (WR) stars. Most of these results are published now in the Proceedings of IAU Symposium N 143 (Van der Hucht and Hidayat, 1991), held in Baly in June 1990. Here we report some new additional results concerning mainly WR stars in binary systems.

This paper briefly reviews the physical properties of symbiotic stars: long-period interacting binaries composed of a red giant primary star and a hot companion. Two types of binaries produce symbiotic optical spectra: semi-detached systems with a main sequence secondary and detached systems with a white dwarf secondary. Semi-detached symbiotics resemble cataclysmic variables and Algol binaries, but on a much larger scale, and undergo dwarf nova-like eruptions. Wind accretion powers detached systems; occasional thermonuclear runaways produce symbiotic novae - distant cousins of classical novae.

Two hypotheses have been put forward for the rôle of binarity in Be stars: (1) All Be stars are interacting binaries. (2) Roughly one-half of the observed Be stars are post-mass exchange binaries with compact companions. Contrary to (1), (2) does not attempt to explain also the existence of disks in Be stars. After the spin-up by mass and angular momentum transfer, the B star somehow has to succeed to form and maintain the disk. Since rapid rotation is only necessary but not sufficient for this transformation, the effect of duplicity would merely be to give more stars the opportunity to become a Be star. Model (1) is not nearly realistic as is also underlined by a new spectroscopic survey for cool companions. The verification of (2) on the basis of the ROSAT All-Sky Survey has just begun; but a serious deficiency of white dwarf companions is already apparent. Binarity currently provides no extra clue on the origin of the Be phenomenon.

We assemble and discuss evidence for binarity in S and MS stars - stars that are enriched in s-process elements. A popular view is that Tc-deficient S and MS stars are not thermally pulsing AGB stars but are mass-transfer binaries. We describe several methods used to test this hypothesis through a search for the putative white-dwarf companion: (1) periodic radial velocity variations, (2) direct observation of the hot companions with IUE, (3) evidence of a hot gas cloud (hotter than a chromosphere) in the system, and (4) evidence of circumstellar dust as revealed through infrared radiation. Results of these methods are compared. All evidence supports the idea that Tc-deficient S and MS stars are mass-transfer binaries, and a large fraction appear to be interactive.