The location of theoretical stellar models in the upper part of the HR Diagram (HRD) depends on a variety of poorly understood physical proc esses which may occur during the evolution of massive stars. Among the most important we find: mass loss, convective overshoot and opacity enhancement for temperatures around 106 °K. The effects of these phenomena have been recently investigated by several authors (Bertelli et al. 1983 and references therein), but, due to the large uncertainties still present in the theoretical formulation of these processes, only parametrized formulae are avaliable for model computations. Since the theoretical dis tribution of massive stars in the HRD turns out to be fairly sensitive to these parameters, the evolutionary scenario for massive stars is still far from being satisfactorily settled. On the other hand, due to mass loss, nuclear processed material is expected to be exposed at the surface of massive stars and then, the comparison between theoretical predictions and observations of the surface chemical composition of these objects can help in understanding their evolution and to set more stringent limits to the mentioned parameters.

It is shown that a moderate increase in the opacity due to heavy elements in models of massive stars, incorporating convective overshoot and mass loss by stellar wind, can remove the well known discrepancy between theoretical expectation and observed frequencies of luminous stars in the HR diagram. These models in fact extend their main sequence band far beyond the classical limit.

P Cygni's evolutionary status is evaluated using its recently redetermined basic parameters (Lamers et al. 1983) to plot the star on the HRD (Figure 1). The theoretical evolutionary track of a 60 M⊙ star passes through P Cygni's error box twice. From a comparison of the time scales along the two parts of the track it is found to be twenty times more probable to find a star in the He-core burning plus H-shell burning phase evolving to higher Teff than in the thick H-shell burning phase evolving to the RSG region of the HRD.

For the determination of ages of OB associations, the traditional approach is to compare isochrones, derived from theoretical models to the observed distribution of stars in the HR diagram (see e.g. Stothers, 1972). However, even a casual look at the HR diagram of the association Per OB1 (Fig.1) shows that it is impossible to fit a unique isochrone to all stars of the group, even by allowing some scatter. We will therefore adopt a different approach and determine the initial mass and the age of each individual star in a given association. As an example, we will discuss Per OB1.

An extensive amount of accurate uvby Hβ data has been obtained for stars in young open clusters. Age spread earlier found within the clusters has been confirmed. Many unevolved stars have been observed. The data for these stars are positioned below ZAMS curves in HR diagrams defined by the Strömgren indices. These unevolved stars form a confined lower envelope for all the stars a little below the “normal” ZAMS curves.

Theoretical isochrones, in the interval 3 to 25 106 y., have been obtained from the grid of stellar models with mass loss computed by Maeder (1980, 1981). They cover the main sequence evolution, the red supergiant region and the first blue loop. These isochrones have been compared to composite colour magnitude diagrams of young open clusters drawn by Mermilliod (1981).

Near-infrared observations of 82 stars in h and χ Persei (NGC 869 and 884) were obtained with the Mexican National Observatory infrared photometer/spectrometer on the 2.1 m telescope at San Pedro Mártir, Baja California. The JHK photometry covers most of the stars in the upper main sequence of the central parts of the clusters as well as the giants and supergiants in a more extended region, with additional L' and M measurements of the brightest members. In Figure 1 the (J-H) vs (H-K) and (H-K) vs (K-L) diagrams are presented. In order to complement the available good quality UBV photometry, we carried out photoelectric measurements of 23 stars in our sample for which only low quality photographic data has been published. These observations were made with the pulse-counting photometer attached to the newly refurbished 1.5 m telescope at San Pedro Mártir.

Neutrino-pair bremsstrahlung rate is calculated in the framework of the Weinberg-Salam theory for the dense matter relevant to the interior of white dwarfs and neutron stars. Ionic correlations both in the liquid state and in the bcc lattice state have been taken into account accurately. It is found that the ionic correlation in the liquid state reduces the neutrino-pair bremsstrahlung rate typically by a factor 2–20 depending on the element and temperature. It is also found that accurate inclusion of the Debye-Waller factor reduces the neutrino-pair bremsstrahlung rate in the lattice state typically by a factor 2.

The evolution of close binaries is discussed in connection with problems concerning mass and angular momentum losses. Theoretical and observational evidence for outflow of matter, leaving the system during evolution is given: statistics on total masses and mass ratios, effects of the accretion of the mass gaining component, the presence of streams, disks, rings, circumstellar envelopes, period changes, abundance changes in the atmosphere.

The effects of outflowing matter on the evolution is outlined, and estimates of the fraction of matter expelled by the loser, and leaving the system, are given. The various time scales involved with evolution and observation are compared. Examples of non conservative evolution are discussed.

Problems related to contact phases, on mass and energy losses, in connection with entropy changes are briefly analysed. For advanced stages the disruption probabilities for supernova explosions are examined. A global picture is given for the evolution of massive close binaries, from ZAMS, through WR phases, X-ray phases, leading to runaway pulsars or to a binary pulsar and later to a millisecond pulsar.

This review addresses the question of what can be learned about the stellar interiors from the study of the tidal effects in close binary stars.

Previous attempts at a detailed confrontation of eclipsing binary data with theoretical models of main-sequence evolution were faced with the choice between data of inhomogeneous (mostly low) quality for many systems (Kriz, 1969; Lacy, 1979) or accurate values of mass, radius, and temperature (or luminosity) for very few systems only (Popper et al., 1970). In addition, more detailed and homogeneous stellar structure calculations for several compositions were needed. Since 1972, a coordinated photometric and spectroscopic programme at our institute contributes to building a sufficient observational basis for such a test. Among published standard models for the range 1–10 M⊙, Hejlesen's (1980) are the most extensive, agree well with other standard models, and are presented in a format suitable for comparison with binary data. Here we can only outline a few salient new results from this study.

When close visual binaries have good and homogeneous data, they can prove to be precision probes of stellar evolution theory.

For normal giants, the only binary system known until recently with completely determined masses was Capella (αAur, GO III+G5III). Being non-eclipsing, its masses are, however, so uncertain (20% and 8%, respectively) that the properties of the corresponding range of theoretical models are considerably more uncertain than the observationally determined temperatures and luminosities. The discovery of the eclipsing nature of TZ For = HD 20301 (Olsen, 1977) thus provides a unique opportunity to obtain absolute parameters of much higher precision.

A 2.5 M⊙ helium star remnant of a Case B mass transfer is evolved to off-centre neon ignition and a second mass transfer phase, Case BB, is explored. The model for the formation of Be/X-ray binaries by mass-transfer dominated evolution in an intermediate mass binary is confirmed.

Six years ago an observational program on supergiant stars using CORAVEL was initiated at Geneva Observatory. About 1500 radial velocities were obtained out of a sample of 181 northern supergiants of F, G, K, M type. Nineteen new SB have been discovered and 16 others are suspected to be SB. The total rate of binary systems among northern supergiants is in the range of 31–38 % (Burki and Mayor, 1983). This value neither depends on spectral type nor luminosity class.

The evolution of a system of 9 MO + 5.4 MO is computed from Zero Age Main Sequence through an early case B of mass exchange, up to the second phase of mass transfer after core helium burning. Both components are calculated simultaneously. The evolution is divided into several physically different phases. The characteristics of the models in each of these phases are transformed into corresponding ‘observable’ quantities. The outlook of the system for photometric observations is discussed, for an idealized case. The influence of the mass of the loser, mli, and the initial mass ratio qi is considered.

We have analyzed the orbital parameters of 33 red dwarf and 17 red giant spectroscopic binaries belonging to open clusters to deduce the time-scale for orbital circularization. The dynamical evolution of BD +23°635, a short period binary member of the Hyades, should result in the formation either of a cataclysmic binary or a WUMa system.

Early studies of apsidal motion in binary stars showed a marked discrepancy between observational results and theoretical expectation (Schwarzschild, 1958; Kopal, 1965). Improvements in theory (Petty, 1973; Odell, 1974; Stothers, 1974) have led to much closer agreement. However, recent observational results (Giménez & García-Pelayo, 1982; Monet, 1980) have provided much more data with which to test theoretical results.

The study of apsidal motions in eclipsing binaries has proven to be one of the best methods to check the internal density concentrations of the stars predicted by theoretical models. During the main sequence phase, we have found a good agreement between the observed apsidal motion rates and computer-constructed stellar models provided that a realistic consideration is made of the evolution between the lower and upper borders of the main sequence (Giménez and García-Pelayo, 1982). An obvious extension of this work is a throughout study of the more evolved evolved systems beyond the TAMS where theoretical models are less accurate and empirical data from different sources are largely needed (see review paper by Zahn in this volume). A preliminary report on such a study is presented.

Consideration of the many types of intrinsic variable stars, that is, those that pulsate, reveals that perhaps a dozen classes can indicate some constraints that affect the results of stellar evolution calculations, or some interpretations of observations. Many of these constraints are not very strong or may not even be well defined yet. In this review we discuss only the case for six classes: classical Cepheids with their measured Wesselink radii, the observed surface effective temperatures of the known eleven double-mode Cepheids, the pulsation periods and measured surface effective temperatures of three R CrB variables, the δ Scuti variable VZ Cnc with a very large ratio of its two observed periods, the nonradial oscillations of our sun, and the period ratios of the newly discovered double-mode RR Lyrae variables. Unfortunately, the present state of knowledge about the exact compositions; mass loss and its dependence on the mass, radius, luminosity, and composition; and internal mixing processes, as well as sometimes the more basic parameters such as luminosities and surface effective temperatures prevent us from applying strong constraints for every case where currently the possibility exists.