In a recent paper (LAUTERBORN, REFSDAL, WEIGERT, 1971) the central He burning phase of 5 M⊙ stars was investigated. It was shown that the structure of such stars is very sensitive to the chemical profile in and above the hydrogen burning shell. The loops in the H-R diagram which such stars often make were for instance found to be very sensitive to changes in this profile. It was also shown that an assumed mass loss of about 10% in the red giant region will suppress a loop completely.

The long range program of the Sproul Observatory initiated in 1937 by P. VAN DE KAMP has been primarily devoted to a detailed astrometric study of the positions of nearby stars from photographs taken with the 24-inch refractor. By now there are 30 years of observations on most stars with parallaxes greater than + O″10 and brighter than the 12th magnitude which are observable at latitude +40°. It follows therefore that there are long series of photographic records of many red dwarf stars.

Observations of the radio emission from the red dwarf flare stars in the solar neighbourhood have been made at Jodrell Bank since 1958. In Australia, observations have been made both of these flare stars and of the early type flare stars in stellar aggregates. Observations in both Britain and Australia have been hampered by the difficulties involved in obtaining radio and optical coverage at the same time.

In the initial Jodrell Bank programme, at a frequency of 240 mhz, between 1958 and 1960, 474 hours of observations were obtained, mostly on UV Ceti, and 13 events compatible with bursts of radio emission from the star were seen, but due to poor seeing conditions at the Cambridge Observatories the correlation between radio and optical flares could not be established, although the rate of occurence of radio events was similar to the reported rate for optical events of a few magnitudes.

In a series of previous papers (ROSINO, 1966; ROSINO-PIGATTO, 1969; PIGATTO-ROSINO, 1971) we have given some results on the systematic search of flare stars in clusters and associations carried out at Asiago with the 90—67 cm Schmidt telescope. We have employed for the survey Kodak 103 a—o or oa—o plates without filter covering a field of 25 square degrees, the flares having been searched with the method of multiple exposures.

YZ CMi (mv = 11ṃ28; B-V = + 1.60; π = 0.̎182 was observed photoelectrically through the B filter of the Johnson and Morgan system on 8 nights during December 1970 to January 1971. The total monitoring time was 22ḥ8 and 3 flares exceeding Oṃ4 in brightness were detected.

I should like to give some information on the activity of the Catania Working Group on Flare Stars constitued by G. Godoli, M. Rodono and myself.

The first purpose of our work is to carry out a sufficient large amount of homogeneous photoelectric observations.

A first series of data, which refer to 1967 and 1968 observations, has already been published in the Supplement of Astronomy and Astrophysics (2, 223,1970).

During last years, after Budapest Colloquium, flare star investigations have been continued at the Crimean Astrophysical Observatory. The purpose of the investigations was to specify spectral features of flares, to research quantitatively spectra of the stars in quiet state and to search for new variables of this type.

The intensive patrol observations of red dwarf stars during last years, carried out in the Crimea, Chile, Catania, South Africa, Armenia, Japan and other observatories led to appreciable increase of number of known UV Ceti-type objects. It permits us to complete the known lists of the variables (1—4). At the same time it is worth-while to define more precisely the criteria to consider a star as the UV Ceti-type variable, and to carry out strong selection of the certain UV Cet-type variables from the objects suspected to be this type variables.

The inception of the present era of interest in rapidly varying stars may be identified with M. F. WALKER’s discovery in 1954 (WALKER 1954) of changes in the brightness of the blue object MacRae+43° 1 (now known as MVLyr), on a time scale of a few minutes. Outside of flare stars, few other stellar objects were suspected to vary on such a short time scale. GREENSTEIN’s (1954) spectra showed that MV Lyr resembles an old nova, which stimulated WALKER to look for light fluctuations in old, recurrent, and dwarf novae and related stars. His observational evidence that all members of these classes show variations of various amplitudes and time scales is summarized in the IAU Colloquium on non-stable stars (WALKER 1957). Most spectacular of his discoveries was the existence of 71 sec periodic variations in the light from the remnant of Nova Herculis 1934 (DQ Her) (WALKER 1956).

The techniques and procedures of time-series monitoring and Fourier analysis which are used at Cerro Tololo to search for luminosity variations in white dwarfs are summarized. Further null results from this program, which are applicable to the problem of defining the region of white-dwarf variability in the HR-diagram, are considered; and the properties of the four known variables are reviewed as possible archetypes of a group.

There are at least five types of variable galactic X-ray sources (i. e. „stars“) known:

Dr. KELLOGG has just described some exciting new observations of X-ray sources made with the UHURU satellite. We shall now move some nine orders of magnitude in wavelength to the opposite end of the electromagnetic spectrum and report measurements of weak radio emission from some of the objects he mentioned. For the detection of weak sources most radio telescopes are not noise limited, but are confusion limited by their low resolution. The aperture synthesis technique minimizes this problem because it enables one to pinpoint the position of weak sources to the order of one second of arc.

It is the purpose of this paper to briefly summarize the results of an eight-year observational program on the Balmer-line behavior of magnetic and related stars. In addition, new data on the interesting 79-minute variable SX Phe will be presented.

The periodic light variability of HZ 29 (= AM CVn) discovered in 1962 by SMAK (1967) was subsequently studied in detail by OSTRIKER and HESSER (1968; hereinafter referenced as OH). The observations of SMAK were confirmed to the blue and ultraviolet light and those of OSTRIKER and HESSER to the integrated light; these revealed light variations in the form of a slightly distorted double sinusoid. This distortion manifests itself that the alternate maxima are broader and, consequently, the alternate minima occur near phase 0.54 P (when phases are counted from minima preceding broader maxima). The over-all amplitude of light oscillations is between 0.02 and 0.05 mag., the period, as determined by OH, is 1051.118 ±0.015 sec.

It is assumed that pulsars are rotating neutron stars surrounded by a strong dipolar magnetic field, the dipole axis being inclined with respect to the rotation axis. It is further assumed that the intense low frequency electromagnetic waves generated under such circumstances can indeed propagate. It can then be shown that charged particles interacting with such outgoing spherical waves are very efficiently accelerated to relativistic energies. During the acceleration which is almost instantaneous the particles emit a short duration pulse which resembles the observed emission characteristics of pulsars.

The consequences of stellar duplicity for variable stars in general are reviewed. Gravitational effects, radiation effects and effects due to gaseous streams and mass transfer are considered. The possibility of tidal interactions, modulating the pulsations of β CMa and δ Sct variables, is among the topics treated in some detail.