Prologue: I rise to address you today with sadness in my heart - sadness which I trust is shared by all those here present; for as is underlined by its subtitle, our beloved colleague Vainu Bappu - past President of the International Astronomical Union, and life-long student of the subject of our Colloquium - is no longer in this world to take his rightful place among us; and, therefore, my remarks which follow can be dedicated only to his memory.

Wide pairs and wide multiple systems have been too much neglected during many years by visual double star astronomers with the argument that only close visual pairs (short periods) may lead to mass-determinations in a relatively short time interval. But mass-determination should not be considered as the only interest of double star astronomy, even if it is of a fundamental nature.

Today, it appears that researches on the origin and the evolution of the wide systems are urgently wanted, not only for the understanding of the evolution of the stellar medium, but also for a better knowledge of galactic dynamics. Some examples are given.

Presently, the main task for double star specialists will be an important improvement in double and multiple star census for all kind of systems: close, medium, and wide. The Hipparcos satellite will probably add some important informations in that respect but ground-based observations also remain of the highest importance (radial velocities, photometry, astrometry, etc.).

A review paper or a lecture like the following one, will best serve ‘its’ conference by giving an overview of the basic facts, and an impartial review of current debates, also by trying to point out some apparently crucial questions whose solutions, we hope, will determine the line of future research.

Because these stars are essentially unevolved, beyond the topic of multiplicity on the Main Sequence looms the fundamental problem of the formation of binary star systems. Thus we are going to concentrate on the following questions: the fraction of stars that are formed in binary and multiple systems, the distribution of mass ratios for unevolved systems, the role of very wide pairs and the smallest known stellar or substellar masses. We will pay special attention to nearby binary stars. On the other hand, we do not have the space to discuss in any detail the binaries in extragalactic systems, in the upper regions of the HR-diagram; they are practically all evolved systems.

Binary statistics, in particular the distributions of mass ratios and orbital periods, are reviewed in an attempt to obtain clues to possible star formation and cloud fragmentation processes. Various observational selection effects which hamper the establishment of the true distributions are discussed. Four different theories of binary formation are compared (fission, fragmentation, capture, and the disintegration of small star clusters), none of which can be ruled out. We conclude that there may be many ways to form binary systems. The dominant mode of binary formation could be ring fragmentation or disc fragmentation depending upon whether the distribution of mass ratios is found to decrease or to increase towards small mass ratios. Future speckle interferometric measurements of a sufficiently large sample of close visual binaries are suggested to settle this important observational question. The present paper is special in that it brings together a wealth of useful information, both observational and theoretical, in one place.

Equilibrium sequences of self-gravitating and polytropic stars including binary stars are computed for various polytropic indexes. We find a fission sequence from an ellipsoidal configuration to a binary by way of dumb-bell equilibrium.

A comparison between results of model calculations and observed properties of close, but detached low mass binaries with Main Sequence primaries shows statistical trends, which seem to support fission origin for some of these objects: the mass-momentum relation, the relation between mass ratio and separation and the relation between mass ratio and synchronisation speak in favour of close initial separations and small mass ratios of the components.

We present IUE UV spectra of the three recent classical novae N Sgr 1982, N Ser 1983, and N Mus 1983, obtained during early decline, transitional stage, and nebular phase, respectively. The line spectra are discussed. Preliminary conclusions concerning CNO over-abundances, shell geometry, and speed classes are drawn.

A catalogue of galactic novae and an atlas of finding charts are under preparation and will be published in 1984. The status of the project is described.

Spectroscopic observations through most of the eclipse cycle of BT Mon reveal the presence of both low and high velocity gas streams. Acceleration through a Laval-nozzle-effect at the inner Lagrangian point of the system and powering of the emission lines through kinetic energy losses of Coriolis deflected and subsequently colliding gas streams are considered as possible mechanisms at work in the system.

I review our current knowledge of symbiotic stars. A great many papers have graced the literature in the fifty years of their study, and many data are available on the spectral variations at optical wavelengths these stars undergo. I do not give extensive references to those data, for previous reviews have done so quite adequately. Rather, I concentrate on the extensive widening of the wavebands within which symbiotic stars have been studied over the past few years, and attempt to synthesise the data into a coherent picture.

Symbiotic stars are most readily explained as interacting binaries, though single star models may still be tenable for some systems. They are made much more complex than most other interacting binaries by the variety of accreting stars, and because gas flows may be highly structured. Moreover, their study is more difficult than that of dwarf novae because the orbital periods are long compared to the activity cycles of the accretion phenomena.

Our data base has expanded enormously with our present spectral Catholicism. But there remains much valuable work to be done with even simple equipment on small telescopes. I suggest in a final section areas for future work.

The properties of cataclysmic variable outbursts are reviewed, and interpreted in terms of dynamical instabilities by the red component, and subsequent evolution of the white dwarf accretion disc subject to a mass transfer burst in the transfer stream.

The radial velocity and profile variations of UV lines of the shell star ζ Tau have been examined in the IUE spectra obtained in 1978-1982. The neutral atoms, and once or twice-ionized ions (except C II, Al III, Si III resonance lines) follow the same velocity variations as in the visual spectra, while the Si IV and C IV resonance lines show a constant negative velocity (~ -50 km s− at the core). The Al III, C II resonance lines and probably Fe III (mult. No. 34) are formed in both regions, i.e., in lowly-ionized and highly-ionized regions and the Si III resonance line is formed in a highly-ionized region.

The photometric variability of CPD-48°1577 in the optical and IR ranges is discussed. The structure and variation of prominent emission line profiles are investigated. An estimate of the distance is given.

A brief description of the variation of the Hα Line profile in γ Velorum and HD 47129 is given.

A method is described for an analysis of deconvoluted stellar spectra, and applied to new spectroscopic observations of the close eclipsing system AE Phuenicis, obtained with the 1.52 m telescope of the Southern European Observatory at La Silla, Chile. Results are presented for the rotational velocities of this variable, together with a proposed model of the system.

VV Orionis is a detached eclipsing binary system consisting of stars of spectral types BI V and B4 V with a period of l.d4854. There is also a third component whose orbital period about the eclipsing pair is about 120 days. In recent years there have been several new photoelectric and spectroscopic investigations of this system, and the results of these are compared. Both the geometric and photometric orbital elements appear to be well determined. VV Ori A appears to be of normal size and mass for its spectral class, but VV Ori B seems to be somewhat smaller than would be expected for a normal B4 V star. Linear limb-darkening coefficients are derived for VV Ori A, and these are in good agreement with theory. This system is of particular importance, because it is only one of very few early-type systems for which reliable limb-darkening coefficients can be expected to be obtained. The contribution of the light of the third component to the system has also been determined, and it appears most likely that VV Ori C is a star of spectral type A3 V.

Photometry of the spectroscopic binary BD + 25°2511 obtained during the 1981, 1982, and 1983 observing seasons shows that it is a variable with a period of about 3.5 days.

On the basis of the most recent data, the fraction of known Wolf-Rayet binaries is 0.22. In the solar neighbourhood (d < 2.5 kpc) this fraction is 0.34

In order to assess the relative importance of massive binary evolution as one of the ways to produce WR stars, the galactic distribution of WR binaries is compared with that of single WR stars using improved intrinsic parameters and new data for the fainter WR stars.

In the galactic plane the increase of the binary frequency with galactocentric distance is confirmed.

In a direction perpendicular to the galactic plane it is demonstrated at all distances from the Sun that the single-line spectroscopic WR binaries with small mass functions have definitely larger |z|-distances than the ‘single’ WR stars and the WR binaries with massive companions. This is consistent with the evolutionary scenario for massive binaries summarized by van den Heuvel (1976). Among the ‘single’ WR stars the fraction of those with large |z|-distances is increasing with galactocentric distance, like the fraction of the known binaries. This implies that among the high-ļzļ ‘single’ WR stars as well as among the WR stars with lower |z|-values many binaries are still to be discovered.

The total WR binary frequency in the Galaxy could be well above 50 %.

Results of a photometric investigation of some photoelectrically neglected, southern eclipsing binaries, are presented for GW Car, X Car, and RS Set. Light curve solutions obtained by the Wilson-Devinney and Wood synthetic light curve techniques are described.