The variability of these objects, almost the brightest ones found in galaxies, is discussed, followed by a description of colour indices and spectra. In the last sections the classification problems and items for further investigation are mentioned.

A correlation between lower excitation potential and radial velocity of violet-displaced absorption components in the spectra of Wolf-Rayet stars is established. The relationship supports the concept of a radially expanding stratified atmosphere in which the expansion velocity increases outwards. The outermost layers of the atmosphere of one star (HD 151932, WN7) display oscillations with a periodicity of about 3.5 days.

The chemical composition of 5 WR stars in Cygnus are considered by using the quasi-nebular variant of Beals' model. On the basis of the general considerations which follow from this model it was concluded that the C/He ratio for the WC stars appears to be more than 10 times higher than for normal OB stars and the N/He ratio in the case of the WN stars seems also to be somewhat higher than for normal OB stars. From the solution of the equations of statistical equilibrium for the level populations, it was found that in the case of the WC stars C/He≈1 and in the case of the WN stars N/He≈0.05–0.1. For the star HD 192163 (WN6) it was found that He/H≈4.2–7.3 and for the other stars He/H≳10.

Emission lines are frequently observed in the spectra of the high luminosity stars, indicating the presence of extended atmospheric envelopes. We are investigating the physical characteristics (differential expansion velocity, density, extension, mass loss) and the origin of these envelopes in relation to the high luminosity of the parent star, and its evolutionary stage. Some results are presented concerning Eta Car, S Dor, HD 34664, P Cyg, AG and HR Car, and two WR stars.

Slow and rapid fluctuations of emission and absorption features of the C III emission line at λ 5696 Å in WC stars are described. They might be attributed to moving clouds in the expanding stellar envelope.

This paper is a review of some of the most important problems on novae. Special attention is paid to the interpretation of recent observational data on novae, structure of the principal envelopes, infrared emission from novae and especially to the problems of magnetic fields in novae.

Arguments are given, suggesting that the most probable type of model is one in which ejection continues after maximum light in the visual region, but with rapid time variations.

The analysis of line profiles in the spectrum of Nova Del 1967 in the nebular stage shows that the polar axis of the envelope is longer than the equatorial one. The polar caps are probably cooler and thinner than the equatorial rings.

Radial velocities were derived from absorption lines of various excitation potentials; there seems to exist a small dependence on this potential.

Spectroscopic observations of Nova Cep 1971 in the visual and infrared regions were carried out at Asiago from July to December 1971. The spectra were characterized at first by the presence of wide emission bands of moderate excitation (H, OI, NI, FeII) accompanied by two absorption systems with mean radial velocity −1300 km s−1 and −2200 km s−1. The velocity was increasing with time. The absorption spectrum disappeared in September and soon the nova entered the nebular stage. The HeI flash was observed in mid-September. In the following months the degree of excitation was steadily increasing. The nebular [OIII] lines became outstanding, even stronger than Hα and the spectrum was characterized by forbidden and permitted lines of relatively high ionization potential (O II, O III, N II, N III, He II).

The profiles of the emission bands were complex from the beginning, with a strong central peak. Later, most of the bands split into two separate components, with radial velocities ± 1360 km s−1.

The light curve was that of a relatively fast nova. The minimum, B∼ 17.5, was attained in the first half of 1973.

The light variations, properties of U Gem binary systems, outbursts of U Gem systems, and evolution and related problems are discussed.

Recent high-speed photometry of Z Cha is mentioned in an addendum.

Basic physical characteristics of nova-like binary systems are similar to W UMa systems. Externally, however, these systems are different – nova-like systems contain a white dwarf as a component, whereas both components of W UMa systems are near the main sequence. The hypothesis is suggested of the origin of contact W UMa systems by means of the fission of the rapidly rotating isothermal helium core of an evolved giant star. The contraction of the more massive component leads to the formation of a white dwarf and consequently to the transformation of a W UMa type system to a nova-like system.

Submitted to Astrophysics and Space Science.

Extensive photoelectric observations of VW Hyi have been carried out during minimum light and eruptions. In the minimum stage VW Hyi shows a periodically repeating hump (similar to that of U Gem) from which an orbital period of has been derived. No eclipses are present. The two types of eruptions known previously for VW Hyi correspond to different physical processes: the short eruptions (lasting ∼4d) resemble again those of U Gem; the minimum hump disappears during bright phases. During the long eruptions (lasting ∼ 17d) a light curve with pronounced peaks has been observed with periodicities of . Significant colour variations with this period are also present. During one long maximum, the repeating feature separated into two peaks. A first attempt has been made to interpret qualitatively the observed facts by an interaction between gas streams from the inner Lagrangian point and ejected matter from the white dwarf component.

The detailed results were published in Astronomy and Astrophysics36 (1974), 369.

This note gives introductory remarks to the three papers mentioned in the references.

There are some arguments that the symbiotic stars are binary, where one component is a red giant and the other component is a small hot star which is exciting a nebula. The symbiotic stars belong to the old disc population. Probably, symbiotic stars are just such an evolutionary stage for double stars as planetary nebulae for single stars.

Short description of the spectral phenomena of V 1016 Cyg.

Light curve and position of V 1329 Cyg in the colour-magnitude diagram are discussed.

The results of UBV photoelectric observations of 12 symbiotic stars have been considered. It must be noted that their light-curves are very different (irregular, regular, of Mira Ceti type) and, in the two-colour diagram (B – V)–(U – B), all these stars are located rather far to the right from the main sequence. The photometric peculiarities of symbiotic stars just mentioned can be explained by assuming that they are binary systems.

Description of photometric and spectroscopic observations of CI Cyg during the 1973 outburst.

A description is given of the spectral changes observed in the symbiotic stars Z And and CI Cyg during the nova-like outbursts of 1968 and 1971–73. The observed features can be interpreted by assuming that transient shells are formed around the hot component of a binary system, imbedded in a small nebula.