Pulsation properties of relevant stellar models are investigated with the linear nonadiabatic method. Short-period variability of FG Sge is interpreted as a pulsation phenomenon.

A model for slow nova explosions is presented. The model consists of a 0.8 Mθ C/0 core and an envelope of 10−4 Mθ with solar composition. The envelope is assumed to have been accreted from a companion. The nuclear runaway produces luminosity close to the Eddington luminosity; this ejects 95% of the envelope. We find

1. I CNO equilibrium burning on a timescale of 105.5 seconds produces enough energy for mass ejection.

2. II The rise in luminosity stops close to the Eddington limit and the outer envelope layers accelerate via the continuous action of radiation pressure.

3. III The mass outflow has two phases: a gentle outflow at the beginning and then a rapid outflow. In both phases we find ṁ ⋍ const., or equivalently, steady state outflow.

4. IV The nova’s “shut-off” mechanism is the exhaustion of the envelope’s mass. In this “slow nova” model it took about 200 days for 95% of the envelope to be ejected and to leave behing a hot white dwarf.

5. V The isotope ratios C12/C13, N14/N15 and O16/O17 are in good agreement with observations.

6. VI The behaviour of LBOL(t) agrees well with observations. Several additional consequences are discussed.

The mass loss from cataclysmic binaries seems an important and worth studying phenomenon for a number of reasons. It is probably enough to mention only two of them:

(a) Whenever we can directly observe the ejected material, determine its amount and the rate of mass loss, as well as its chemical composition (this being the case of the expanding envelopes of novae), we are getting a good insight into the basic physical mechanisms responsible for the observed phenomena.

(b) The mass loss (together with the mass transfer) and the loss of the orbital angular momentum are related directly to the dynamical evolution of a binary system and - indirectly - to the evolution of its components.

In 1925 a photographic search for new variable stars was begun at the Remeis-Observatory in Bamberg. Initially the sky patrol covered only the northern hemisphere, but in 1964 it was also extended to the southern sky. At the individual observing stations, the sky is systematically photographed with several wide-angle patrol cameras which are attached to the same mounting, and which have f/6 Tessar lenses of 4-inch aperture. Each camera covers a 13-by-13 degree field. The plates are usually exposed for one hour and a photographic magnitude of 14m is reached.

The peculiar emission-line object V 1329 Cyg was discovered by Kohoutek (1969), who detected in his objective-prism spectrograms an increasing brightness and the presence of emission lines. High dispersion spectra revealed more than 200 emissions of hydrogen, helium, ionized metals and forbidden lines of oxygen, neon, etc. (Crampton et al., 1970; Andrillat, 1970). The object exhibits the composite spectrum of a hot early-type component and a cold M-type star. Its time evolution in a two-colour diagram is seen in Fig. 1.

Photometric and spectroscopic observations of RY Scuti are presented. These da ta indicate an advanced stage of this β Lyrae-type massive system, whose further e volution could lead to a SN-II event with the formation of a system like Cyg X-l.

It is arpued that late type W UMa binaries undergo mass ejection, mainly through the second Lagragian point, which causes a steady decrease of their mass ratio m2/m1. This makes it reasonable to believe that the W UMa stage is a state of transition between T Tau and normal main sequence stars.

About 6000 times of minima for 120 W Ursae Majoris stars were collected by Mrs. Z. Frasińska and the writer in the Cracow Astronomical Observatory. From this material 15 O - C diagrams (fig. 1) with large number of minima were taken into consideration. Each of them covered more than 30,000 epochs. The analysis of O - C curves leads to the following conclusions:

1. 1. The periods of all well-observed W UMa stars are variable.

2. 2. There are no “pure” parabolae in well-observed O - C curves.

3. 3. The periods change in a rather short time and then remain constant for 5,000 - 25,000 E (fig. 2).

4. 4. The number of increases and decreases of periods is approximately the same. The mean absolute change of period of 15 investigated stars amounts to 0.d00000365 = 0.P0000105. (The mean increase of period = 0.d00000386 = 0.P0000109, the mean decrease of period = 0.d00000349 = 0.P0000102).

5. 5. The mean absolute change of period for 15 investigated stars was plotted versus parameter q (mass ratio = m2/m1) (fig. 3). Systems with almost equal masses of stars have a tendency to show violent changes of period.

In 1967 Wilson and Barrett (1968, 1970) discovered that some long period variables, very red and apparently very late type stars, emit OH microwave line emission that is especially strong in the 1612 MHz line at 18 cm. At present some 65 such OH-emitting stars have been identified - for a recent compilation see Bowers and Kerr (1977, M giants) and Baudry et al. (1977, M supergiants). Several stars show maser emission in H2O and SiO as well. At this colloquium Winnberg will review the present status of our knowledge of these stars.

Some infrared counterparts of Type II OHAR sources have been observed between 1µ and 30µ. In spite of their having very similar OH emission spectra, their energy distributions differ considerably among themselves from “flat” (1. 25µ to 4. 9µ) to “steep” (2. 2µ to 20µ). The energy distributions are compared with more familiar objects.

Observations at 1612 MHz around the Galactic Centre have revealed the existence of 43 OH/IR stars. Sources within 6° longitude from the Galactic Centre appear to be associated with the central parts of the Galaxy. They have a large velocity dispersion and a mean radial velocity of 0 km s−1, indicating that there is no net rotation around the Centre. Within 2° from the Centre there is a strong relative increase in the number of sources with a velocity separation between the two emission peaks of less than 30 km s−1. This is discussed in terms of a possible interaction between the circumstellar shell and the ambient medium.

Substantative mass-loss resulting in appreciable circumstellar dust envelopes is common in late-type stars. The evolutionary history and physical state of a cool star determine the chemistry within the outer stellar atmosphere mirrored by the molecular and particulate material present in the envelope. The observational consequences of this debri determined by moderate spectral resolution (λ/Δλ ~ 50-100) infrared spectrophotometry are reviewed. Significant information is provided by observations of the emergent energy flux of both the cool stellar photosphere and of the circumstellar dust envelope. While most of the infrared features observed in late-type stars can be plausibly identified, an as yet unknown emission mechanism marked by strong infrared band structure is known to be operative in a variety of infrared sources. The observed range in shell optical depth in both M and C stars suggests that mass-loss occurs to some degree throughout late stellar evolutionary phases and that occasional periods of high mass-loss are not uncommon.

Radio observations of spectral lines from OH, H2O, SiO and CO have put the spot-light on late-type stars and revived the interest in them. Especially they have helped the understanding of the outer envelopes of these stars and of their mass loss. They also have revealed late-type stars that are hidden from observation at optical and near-infrared wavelengths by their dense surrounding shells of gas and dust. The strong maser lines from OH, H2O and SiO have additionally given us an excellent tool in mapping the distribution of late-type variables throughout the Galaxy. On the other hand, the interpretation of the maser amplified lines in terms of a model is difficult. Very-long-baseline interferometry (VLBI) of these lines might be a way of studying the geometry and kinematics of the envelopes but there are a few problems connected with these observations which I will point out in this paper. However, recently “thermal” microwave lines of SiO and CO have been observed from late-type stars. These lines might be easier to interpret regarding physical processes in the outer circumstellar envelopes. As so often when a new field is opening up in astronomy the solutions to several old problems have been found but at the same time many more new problems have been brought up.

We consider further the question of the origin and evolution of metal-rich RR Lyraes (MR-RR’s) in the solar vicinity, continuing the discussion begun by Taam, et al (1976) who showed that MR-RR’s descend from red giants of Eggen’s old disk population. If an old disk giant (ODG) is similar to a giant in a galactic cluster like NGC 188, then it must lose~0.5 M⊙ if it is to pump the HR-RR domain; thus the probability that an ODG will metamorphose into an RR Lyrae is about 1/200th the probability that its metal-poor (halo) counterpart will do so, else the sky would be full of bright MR-RR’s.

Mass-loss from red-giant stars has been proved by optical observations of violet displaced absorption cores in strong metal lines of low excitation (Deutsch, 1956) long before vast circumstellar shells around the most luminous stars were detected by infrared radiation from circumstellar dust (Gillet et al. 1968, Woolf and Ney, 1969), by intrinsic polarization of star light (e.g., Serkowski, 1966) and by molecular microwave emission (Wilson and Barrett, 1972). Hot expanding coronae which emit UV lines of highly ionized metals were detected only recently in G and K giants (e.g., β Gem, Gerola et al., 1974).

Since Deutsch’s (1956) discovery that in α Herculis the displaced circumstellar absorption lines common in the spectra of late-type giants and supergiants really represented a loss of matter, the evidence that this is a very extensive and common feature among all late giants and supergiants has become very persuasive. To the extensive optical spectroscopic evidence for mass loss from these stars originally obtained by Deutsch and subsequently by others, we may now add observations of infrared excess from dust in circumstellar envelopes (Merrill 1977), radio molecular line radiation from expanding shells (Winnburg 1971), thermal and possibly 21 cm radio radiation from ionized and neutral hydrogen respectively (Smolinski et al. 1977; Zuckerman 1977) and the direct photographs of the envelope of α Ori in the light of one of the potassium resonance lines from scattered stellar radiation (Bernat et al. 1977).

Observations show that long period variables (LPVs) as well as many other giant and supergiant stars are losing mass extensively. As evidence of mass loss one can mention the presence of circumstellar envelopes around LPVs. Such envelopes are inferred from infrared excesses (Gehrz and Woolf, 1971; Reimers, 1975) and maser radiation. P Cyg type profiles are observed in the spectra of some LPVs (Sanner, 1977). The increased brightness of the companion of Mira (a white dwarf) has been interpreted as the result of accretion of matter ejected by the primary (Shklovsky, 1976).

V 1016 Cygni is a peculiar variable star, whose brightness increased from about 16 mag in 1963 (previously it was known to have a LPV spectrum) to about 11 mag in 1967. Since then it has been nearly constant at maximum. Our last photometric observations, from May to August 1977, indicate that the star is still in its protracted maximum at a constant brightness. We have measured B = 11.1 and V = 10.8 (± 0.2 mag s.e.).

The long-period variable giants with maser emission (maser/LPV stars) can be divided into two groups. The stars in group 1 have color indices I (0.84μ) - K (2.2µ) < 6, those in group 2 have I - K≥6. Fig. 1 gives the velocity structure of the maser lines for both groups. I have included only those maser /LPV stars for which the radial velocity of the star is known. In most cases this is the mean velocity of the outer OH peaks (Winnberg, this Colloquium), but for R Leo and R Cas the stellar velocity is found from the ground-vibrational state SiO emission (Reid and Dickinson, 1976). The abscissa of fig. 1 gives the velocity relative to the stellar velocity and the verticale scale gives the number of stars that show emission in a specific line at the relative velocity indicated. Note that the “line profiles” obtained in this way are not real mean profiles, because I only counted the presence of emission, weak or strong.

The light-curve between 1964 and 1977 for the variable M2II giant HD 65750 = V341 Car is derived from 77 photographic and 83 photoelectric UBV measurements and analyzed. It is concluded that the light variations of the star are irregular and due to variable extinction in the circumstellar nebula. The appearence of the visible reflection nebula IC 2220 into which HD 65750 is embedded, has been found to vary on a time scale of four years.