I would like to start with a short historical introduction. In 1938 thermonuclear reaction rates for hydrogen burning became available. This made it possible to fit a convective core into the point source model of stellar structure integrated by Cowling three years earlier. The free parameter in the fitting process could be fixed with the thermonuclear reaction rates, the first realistic stellar model for a massive main sequence star was constructed! After the war electronic computers became available, and one was able to do more complicated models like those on the lower main sequence, like realistic models for the sun with its helium enriched interior, and one tried already to follow in time the exhaustion of hydrogen in the central regions of stars numerically. There was not too much progress for stars at the upper end of the main sequence. As soon as the stellar model tried to leave the main sequence and to march towards the region of the red supergiants the methods known at that time failed to produce models. For less massive stars the exhaustion of hydrogen could be followed up more easily and, in 1955, the great paper by Hoyle and Schwarzschild came out, which showed how these stars from the main sequence move into the red giant branch and move up parallel to what we now call the Hayashi line (which was not yet known at that time). But when helium started to burn the methods also failed.

The brightest individual objects in extragalactic nebulae are the Hubble-Sandage variables. They were first investigated by Hubble and Sandage (1953). Observations by Tamman and Sandage (1968) and Rosino and Bianchini (1973) followed. The main characteristics are high luminosity (L/Lʘ ≈ 105), blue color indices, F type spectra and irregular variability.

Bath (1979) has suggested that the Hubble-Sandage variables contain an accreting main-sequence star with a Roche Lobe filling companion in a wide binary system. Based on this model we derive theoretical color indices for disks and determine the mass in the disk for different mass accretion rates. Further we discuss an instability of the disk which could explain the change in the color index observed for Var A in M33.

We summarize the methods now available to detect the presence of companions to Cepheids, and then we analyze the results of the application of the recent photometric method proposed by Fernie (1980). If applied to the set of VBLUW observations of southern Cepheids carried out by Pel (1976), this method gives a frequency of about 25%, which is consistent with the more recent spectroscopic determinations (Gieren, 1981). The importance of the IUE satellite to directly observe the U flux emitted by the B-companion is emphasized.

The importance of the search for pulsating stars of the δ Scuti type in multiple stellar systems is emphasized, in particular for the following objects: the evolutionary status of these systems, the interconnection between multiplicity and pulsation (Frolov et al., 1980), a verification of the theory of the turbulent mixing (Vauclair, 1976). A list of possible pulsating companions in visual and spectroscopic systems with foreseeable period and amplitude is given. Some preliminary results of a spectrophotometric survey begun at the Observatories of Milano - Merate and Bologna - Loiano are also given.

Taking into account the possible influence of external factors such as the orbital motion in binary systems and the rotational velocity on the pulsation of δScuti stars, it may be possible to explain some properties of these variables.

We present the main results of a study of the observed internal structure constants, k2, for a wide set of eclipsing binaries. From the analysis of the variations in relative positions of the eclipses and the comparison with different theoretical models, we could deduce that the discrepancy, previously reported by several authors between theory and observations, is no longer supported. Moreover, a strong correlation has been found between the evolution of the parameter k2 and the gravity at the surface of the star, g.

Investigation of the variability of the orbital periods of eclipsing binaries is important not only from the evolutionary point of view but also for detecting additional components in them. Systematic study of the period changes in binary stars was started by Plavec (Plavec et al 1960) more than twenty years ago. Work on the same lines for 20 detached systems was reported by Herczeg (1980) at the I.A.U. Symposium No. 88 held in Toronto two years ago. Here we describe the study of 22 systems carried out by us at Hyderabad.

The discovery and study of stellar associations by Ambartsumian (1947, 1954 a) proved two fundamental points regarding the star formation process, e.g., Mirzoyan, 1976. One of them is that the formation process still continues in the present evolution of the Galaxy, and this thesis is not disputed now. According to the second, stars form as physical groups of multiple stars and star clusters, which become stellar associations during the initial phases of their evolution.

It has been found that the close spectroscopic pair of this system consists of a G type primary and a late type secondary, which most likely is a T Tauri star. As such, the system is semidetached and a mass transfer takes place between the components.

In the late stages of stellar evolution, relativistic objects are formed, such as neutron stars or black holes. These relativistic stars possess a strong gravitational field, therefore their structure and their space-time geometry can be described only in the frame of GRT (Zeldovich and Novikov, 1971; Misner, Thorne and Wheeler, 1973). For this purpose, the following four-dimensional interval is used (spherical gravitational field), (Zeldovich and Novikov, 1971).

Zdenek Kopal has kindly invited me, and I have accepted, to “instruct the theoreticians on known facts”. He also asked me to express my opinion on the relative evolutionary stages of components.

I am essentially an observing astronomer, occupied with stars in our immediate neighborhood, say within 10 or at most some 25 parsec, i.e., the lower main sequence and the white dwarf degenerate branch.

I hope that I may perhaps contribute by surveying and reporting some relevant data. I shall touch on a number of topics, limited because of selection and lack of knowledge. My contributions to binary stars lie in the realm of parallaxes, mass-ratios and masses, – and for the past half century, perturbations, interpreted as unseen companions, stellar and otherwise. I shall briefly report on some results, and I shall be wondering and hoping that some trace of stellar evolution may possibly be present in these results. After having witnessed for more than half a century my own astronomical evolution, the time has come for me to become more aware of theoretical, evolutionary and cosmological aspects of the cosmic material, I have been playing with so long.

The space distribution of some small dense clouds with point-like IR sources, resembles the clustering of young OB stars. It can be assumed that such objects contain heavy obscured high-luminosity stars on the ZAMS. From the comparison of infrared and radio data it follows that in typical cases, only one B star is the source of the radiation of the cloud. The total mass of the cloud is of the order of one solar mass. If the individual fragments of the cloud are gravitationally unstable, then in the later stage of the evolution only low-mass stars are formed. One can therefore expect that young OB stars are most frequently accompanied by low-mass pre-Main Sequence stars.

By a comparison of Trapezium-type multiple star systems in the Abastumani Catalogue and the famous Catalogue of Stellar Associations and Clusters, it is confirmed that the great number of Trapezium-type multiple stars, which belong mostly to spectral classes 0-B2, are found in associations and clusters.

In 13 T-associations, 120 Trapezium-type multiple systems and 182 common multiple and double stars could be detected.

Based on present-day observational data and the author's photographic observations, the kinematic of Trapezium-type multiple stars of spectral class 0-B2 is studied.

This contribution is concerned with the origin of binary stellar systems (cf. Abt 1977, Huang 1977). The hierarchical fragmentation scheme of rotating interstellar clouds, proposed by Bodenheimer (1978), is combined with the role of magnetic fields in the early stages of star formation (Mouschovias 1978, Dorfi 1981). The possible influence of the local strength of the mean interstellar magnetic field on the local fraction of binary systems in a galaxy is stressed.