Four years ago, in 1975, we met in Cambridge, England for our first IAU Symposium devoted entirely to binary stars. Most of the talks given there were theoretical, and at the end some of us felt that it would be appropriate to organize yet another Symposium soon, this time oriented more towards observations and their immediate interpretation. This is why we have come together here in Toronto. Four years is not a long interval of time, and few disciplines of astronomy repeat their Symposia on such a short time scale. While the need for another Symposium was clear to the binary stars investigators, it is not so obvious to other colleagues. We must therefore be very grateful to the Executive Committee of the International Astronomical Union for their great understanding of our needs. This understanding went far beyond approving and sponsoring this meeting: We have received a substantial financial assistance, which all went to the support of the travel expenses of some of the participants. Many more travel grants were possible thanks to a most generous support by the University of Toronto. I would like to thank both institutions.

Although the formation of close binaries has been the subject of numerous investigations in the last 100 years (for a recent review, see Tassoul 1978), we cannot yet claim decisive observational support for any theory, a state of affairs probably reflecting insufficient development of the various theories, rather than a failure to propose the correct mechanism(s). If this is indeed so, we can look forward to significant progress soon, since numerical techniques now allow us - albeit crudely - to follow through to completion binary formation by the two most likely mechanisms, fission and fragmentation. (Herein a binary will be described as having formed by fission if it results from the bifurcation of a rotating protostar as it contracts quasi-statically towards the main sequence and by fragmentation if it results from a rotating protostar's break up into two or more components during, or immediately following, a phase of dynamical collapse).

A catalog of physical parameters of about 1000 spectroscopic binary stars (SB), based on the Batten catalog, its extensions, and newly published data has been compiled. Masses of stars′ components (M1 and M2), mass ratios of components (q = M1/M2) and orbital angular momenta are computed, wherever possible. It is probable that the initial mass function of the primaries is non-monotonic and is described only approximately by a power-law. The distribution of q of double-line SB has a sharp maximum near q = 1, that of single-line SB, near q = 0.3. SB with 1.5 ≲ M1/M⊙ ≲ 10 and semiaxes of orbits a ≲ 10 R⊙ probably do not form. A number of assumed “initial” distributions of M1, q and a were transformed with the aim of obtaining “observed” distributions taking into account the observational selection due to the luminosities of the components, their radial velocities, inclinations of the orbits, and the effects of matter exchange between the components. The best agreement with the observations is obtained for the combination of a power-law initial mass function ξ(M) ≃ M−2.35, a distribution of q with a maximum near 1 and a distribution of semiaxes with dN/da ≃ 1/a.

The analysis of the period distribution of eclipsing and spectroscopic binary systems shows the presence of some secundary maxima, which cannot depend on selection effects. These secundary maxima are mainly due to late type stars, as can be seen from the distribution curves for eclipsing binaries of various spectral types. The average separation of the components (in units of the sum of stellar radii) increases with the spectral type from 0 types to late B types, remaining almost constant for later spectral types.

It has been demonstrated (Zahn 1966 a, b, c; 1975, 1977) that turbulent viscous dissipation of tides can brake the rotation of stars having convective envelopes, while radiative dissipation can do like-wise for those having radiative envelopes. The time scales for the former are very small whereas those for the latter depend sensitively on the ratio r/R where r is the radius of the star and R, the binary separation. For most systems these turn out to be much less than their main sequence lifetimes. This fact affords a natural explanation for the preponderance of synchronous systems.

A simple examination of Table I which lists the 26 positively detected “active” radio stars reveals that 21 are interacting binaries. Thus, on face value, membership in a close binary system would appear to be an important factor in producing conditions which favor stellar radio emission. However, this conclusion ignores the considerable biases which have characterized most of the surveys (c.f., references in Wendker 1978) made to date wherein selection criteria such as binarity itself, evidence for mass exchange, strong magnetic fields, optical activity, and similarity to previously detected systems have been cited. In very few cases (c.f., Gibson 1977) have complete samples (i.e., a uniform survey of all stars in a particular class to a given luminosity limit) been observed. Therefore, it is difficult to examine the role of binarity in stellar radio emission and draw conclusions which have statistical support. However, the body of radio star data does include observations of about 700 objects and thoughtful analyses of a carefully selected subset should reveal trends which, if they do not lead to firm conclusions, should at least point out the direction for future study.

The recent launch of the International Ultraviolet Explorer (IUE) satellite and its spectrographic instruments (Boggess et al. 1978a,b) suddenly made accessible large numbers of diverse types of binary systems. These range from the luminous hot supergiants with compact companions that produce copious amounts of X-rays to the dwarf solar like stars of W UMa-type. For systems such as these and many others the IUE is a new and unique tool with which to probe the structure of extended stellar atmospheres, the presence and acceleration of a stellar wind, and the appearance of stellar chromospheres and coronae.

The recent installation of a solid-state 1024-element silicon photodiode array detector (Reticon) at the coude focus of the 2.7 m McDonald Observatory reflector has greatly extended its limits of observation for binary and multiple systems which have weak and/or broad-lined components. This detector can produce extremely high signal-to-noise ratio observations and has high quantum efficiency over the wavelength region 3000-11000Å. The observational programs of three users of this device are described below.

A program of Hα and Hβ narrow- and intermediate-band photometry of close binaries has been carried out at Biruni Observatory since 1977 and at Villanova University Observatory since 1971. The advantages of this type of photometry for close binary work are discussed. Stars which have extensive coverage are listed and results for Cyg X-1, WZ Sge, W Ser and HR 1099 are briefly discussed.

Polarization caused by scattering of starlight on gaseous extrastellar material in close binary systems is reviewed. A simple physical derivation is given to illustrate how in principle, variations synchronous with the orbital period of the Stokes parameters of the linear polarization can yield the orbital inclination and other parameters. High resolution multichannel spectropolarimetry across the emission line profiles of binaries is discussed as a new technique in studying the physics and kinematics of gaseous streams and stellar winds. The methods have application to a range of binary objects including systems like Algol and Beta Lyrae, X-ray binaries, Of and Wolf-Rayet binaries, VV Cephei stars and symbiotic stars. Some new observational results are presented.

The polarimetric variation from a binary system with an eccentric orbit, thus non-corotating, are calculated and the effect on determining the system parameters is discussed, relative to the circular case.

A new method of light curve analysis is introduced for systems involving an atmospheric eclipse. Occultation or partial eclipses may be treated, with or without the presence of transparency effects. Besides admitting an arbitrary degree of transparency to the eclipsing component, an arbitrary law of limb darkening may also be given the eclipsed star.

The method has successfully been applied to the study of narrow band continuum light curves of V444 Cygni obtained by Cherepashchuk and Khaliullin. Primary and secondary minima are examined separately.

Determinations of absolute dimensions of main sequence binaries are no longer useful unless random and systematic errors are kept appreciably below 10%. Some essential conditions for achieving this are reviewed.

A comprehensive study of period changes in eclipsing binaries is being carried out at the University of Oklahoma. The survey of available information is supplemented by observations obtained at the University Observatory, at Kitt Peak and Cerro Tololo observatories, and, as part of a cooperative program, at Ankara University. The present review reports the behaviour of 20 systems classified as detached, some of which do show noticeable variations of the period.

By estimating the orbital inclinations of non-eclipsing detached close binaries with Ap spectra, a marked statistical preference is obtained on the frequency distribution of the inclination which suggests that the abundance anomalies of Ap components tend to concentrate towards the stellar polar region.

From a search through the literature 174 close binaries with completely known absolute dimensions are sampled. Distinction is made between systems before and after mass exchange, giving resp. 100 and 40 systems (a third group contains the systems not definitely belonging to these two). Mass, period and mass ratio distributions and relations of the group of “unevolved” binaries (i.e. prior to mass exchange) are transformed into corresponding distributions and relations of evolved binaries. The transformations are based upon the M1 f = g (M1 i) relation derived from an extended set of published theoretical computations on the evolution of close binaries. Final masses resulting from the same initial mass are averaged. Equations are derived for the cases A (for all masses), B1 (M1 i/Mo < 2.8), B2 (2.8 < M1 i/Mo < 9) and B3 (M1 i/Mo > 9). For the changes of the period due to angular momentum loss the formalism of Vanbeveren et al. (1979) was adopted. The following characteristics of the system after mass exchange are computed: M1 f, M2 f (and qf), Pf. Three different modes were applied for the mass loss from the system:

a) conservative case (mass and angular momentum of the system remain constant), called C.

b) non conservative case with 50% of the transferred mass leaving the system with a small or a large angular momentum loss (resp. called NC51 and NC53).

c) non conservative case with 100% of the transferred mass leaving the system with a small or a large angular momentum loss (resp. called NC101 and NC103).