UBV observations of RCB stars with δ⩽30° were made at La Silla, Chile in the summer of 1972. Of 26 stars, 5 were below maximum brightness, including R CrB and RY Sgr. On the basis of a comparison of the dispersion in observations of the RCB stars and nearby non-variable comparison stars, 9 RCB stars were found to show significant variation at maximum brightness. They are W Men, UX Ant, UW Cen, DY Cen, AE Cir, S Aps, RT Nor, RZ Nor, and V CrA. The amplitude of these variations ranges from to and tentative periods from 19 to 54 days and > 90 days for S Aps. Similar variations at maximum are already known for R CrB, RY Sgr and XX Cam. Of the 5 stars which showed no significant variation at maximum, DZ And, Z Cir and V 973 Oph (see Orlov and Rodriguez, and Feast, this Symposium) are not RCB stars and SY Hyi is a questionable RCB star; only Y Mus is listed as a definite RCB variable.

4 RCB stars in the Large Magellanic Cloud were observed: W Men, SY Hyi, HV 5637, and HV 12842. Absolute magnitudes at maximum of and respectively were derived. When plotted on a colour-colour diagram, most of these stars show an UV-excess for average colours at maximum. In the H-R diagram, they fall in a region where one would expect to find stars which are losing mass.

The paper contains preliminary results of theoretical investigations of the light curves of RCB variables. A circumstellar dust envelope is assumed to be the source of the great visual opacity as well as the infrared and violet excesses. The processes of grain formation and the interaction with the radiation field are discussed briefly. The results of a laboratory experiment concerning the problem of the violet continuum are also described.

Spectroscopic and polarimetric observations of the RCB variables DZ And, R CrB and XX Cam are described. DZ And is not a RCB-type star.

The observations of R CrB and XX Cam obtained in 1971–1973 are described. Semiregular light variations with an amplitude of and a period of about 30 to 40 days were found during maximum brightness. In addition, fluctuations up to for about 2 h were observed.

The optical properties of circumstellar graphite particle shells are considered. In the case of the optically thin shell (e.g. the shell of a RCB star after minimum), the temperature of the grains is calculated. It is shown that the dust shell always has a sharp inner border. The RCB phenomenon may be considered as a sudden moving of this border toward the star and the consequent increase in its optical thickness.

By combining lists of carbon stars with the catalogue of open clusters (Alter et al., 1970), a list of variable carbon stars, being probable members of clusters is given. The number of stars is too small to derive reliable statistics on cluster membership. Too few radial velocities are yet available. Photometric criteria are still too inaccurately known.

Supergiant infrared stars with OH maser emissions, like NML Cyg, VY CMa, and VX Sgr, are discussed assuming a model consisting of a star with a photospheric temperature of about 2000 K and a radius of R ∼ 2 × 1014 cm surrounded by a gas and dust shell. A possible similarity with OH Miras is mentioned.

This paper gives an introduction to the use of autocorrelative analyses in the case of irregular and semi-regular brightness variations.

It is pointed out that observational work on Cepheids in the R, I photometric system offers a number of advantages: the stars are generally brighter at these wavelengths and so can be more easily and accurately observed; effects of binary companions, which are usually earlier type stars, are minimized; reddening is less, can be more easily determined, and is less liable to differing extinction laws than in the UBV system; line-blanketing problems are smaller; the range in (R — I)0 is probably less than in (B — V)0, and the importance of the colour term in the P-L-C relation thus reduced.

Available R, I data for classical Cepheids are gathered from the literature, and the P-L, P-C, and P-L-C relations investigated. It is found that the preliminary relations

represent the data to within ± 0.13 mag. in 〈MR〉 and ± 0.02 mag. in 〈R–I〉o.

There is some indication that the intrinsic colours of EV Set, U Sgr, and S Nor are bluer than hitherto believed, conceivably due to circumstellar reddening.

A photoelectric area scanner (Franz, 1970) was used to carry out photometry of the separate components of the double Cepheid CE Cas, a binary of 2.5″ separation located in NGC 7790. CE Cas A was found to undergo rapid fluctuations as well as a slow change of its brightness near minimum phase, while its brightness at maximum has remained the same over more than 10 yr. The absence of the effect from the observations of CE Cas B and evidence for its presence in earlier observations by Sandage and Tammann (1969) serve to establish the reality of this phenomenon. Manifestation of this effect during early phases of this investigation had led to the erroneous conclusion that CE Cas A had undergone a significant change of period in recent years (Franz, 1972).

This paper contains a rediscussion of the period-luminosity relation of type I-cepheids. Special attention is paid to the radius and its variation.

Using rich photometric material on the Cepheids in the Galaxy and the Large and Small Magellanic Clouds, the structure of the instability strip in the HR diagram is investigated. The method of the δ-δ diagrams is used, δ being the difference of certain Cepheid characteristics from the means for a given period. The Cepheids in the three galaxies, studied separately, show the same relationship: the light amplitude increases toward the low temperature limit of the instability strip and is also related with the temperature at maximum in the sense that Cepheids with a given period and fixed mean temperature have a greater light amplitude for a higher temperature at maximum. Our results do not coincide with those by Sandage and Tammann, probably because they have studied the Cepheids in the different star systems as one single complex.

The significance of variable stars as members of open clusters is outlined. The results of a search for such stars are briefly mentioned.

I want to show two slides with the evidence of the presence of the λ 10830 Å HeI absorption line in the spectrum of the classical Cepheid ζ Gem. In Figure 1 you can see the profile of the blend near λ 10830 å (dispersion 48 Å mm−1); this blend consists of lines of neutral silicon, helium and atmospheric water. The monotonous phase changes of the profile are easily seen.

The frontier of our knowledge of δ Scuti variables is reviewed with emphasis on the following topics: (1) pulsation in specific clusters; (2) rotation and pulsation in different clusters; (3) metallicism and pulsation; and (4) periods and pulsation modes of δ Scuti stars.

It is widely accepted that pulsating variables with periods less than 0.2 day fall into two distinct groups. These defined by Kukarkin et al. (1969) as, firstly,

RRs:RR Lyrae type variables with the period not exceeding 0d.21 (dwarf Cepheids). Belong to the population of the disk, are absent in clusters. Their luminosity is 2m–3m fainter than the luminosity of RRab and RRc stars. A typical representative – SX Phe.

and secondly,

δSct:δScuti type stars. Pulsating variables of spectral class A (late sub-classes) and F, the amplitudes of light variation do not exceed, as a rule, 0m.1 (rarely up to 0m.3). The form of light curve strongly varies usually. According to many characteristics resemble dwarf Cepheids, but differ from them by the small amplitudes. Are met in the Hyades-type clusters. Similar to the RRs type stars their periods do not exceed 0d.2. A typical representative – δSct.

In this paper the pulsations of δ Sct star models are compared with observational data. The locations of the blue edges of the instability strips were computed as a function of helium abundance. Linear non-adiabatic pulsations were computed following Dziembowski's programme.

Arguments are presented that the AI Velorum variables are about 0.2 M⊙ objects evolving with a roughly constant luminosity toward the white dwarf stage.

Photometric and spectroscopic observations were made by Oskanian and Terzan (1974) on the 40-cm telescope at Byurakan Observatory and by Morguleff et al. simultaneously at the 80-cm photometric telescope and the Coudé Spectrograph of the 152-cm telescope of the Observatoire de Haute Provence.

This review discusses the spectral classification, the absolute magnitude, the position in stellar systems, the physical properties and the evolution of WR stars.