The study of stellar evolution can be undertaken either from a purely theoretical point of view or from a more observational approach. The present standpoint is the second one. It starts from the concepts of stellar evolution in associations and open clusters and from stellar flare activity. Statistical considerations show that flare activity is a regular stage in the evolution of stars through which all the dwarf stars go.

Since Gershberg's (1970a) very thorough review of the subject, numerous papers dealing with the collective properties of flare stars have appeared (Petit, 1970; Kunkel, 1970d, 1973; Ambartsumian and Mirzoyan, 1971; Gurzadyan, 1971; Oskanian and Terebizh, 1971a, b; Gershberg, 1972; Veeder, 1973). The basic features of flare activity can be taken as known, then.

The UV Cet-type star flares and the chromospheric flares of the Sun have many important common features. Both types of events are very transient; they occupy only small parts of the stellar surfaces; in both cases the optical as well as the radio emissions of the flares have strong similarities in temporal and spectral characteristics. Evaluations of electron temperatures, electron densities and the general character of mass motions within radiating gas in the stellar flares also display similarities to solar flares. The solar and stellar atmospheres where the flares develop have many important common characteristics too: the existence of hot chromospheres and coronas above much colder photospheres, the absence of noticeable dust envelopes, the photospheres patched with dark short-lived spots, and some periodicity in spot appearance over several years. The facts permit one to identify the processes which take place during separate solar and stellar flares and which are responsible for the solar and stellar activities in total. From this point of view some general problems of physics and evolution of red-dwarf flare stars are discussed.

Flare stars in the solar neighborhood and in associations belong to the same class of objects but might represent different evolutionary stages. In this report, fundamental contributions, based on observations by various groups, on flare stars ranging from extremely young ones to dMe flare stars are summarized.

Four years ago I examined the flare incidence data then available for UV Ceti and V1054 OPH (Wolf 630) and concluded that no evidence could be found for periodic or secular variation in the levels of activity (Kunkel, 1971). Variations greater than 0.3 mag. (rms) should have been found.

Recent photoelectric observations made at Catania Observatory show a high occurrence of short-lived faint flares on the Hyades member II Tau (H II 2411). Its activity level turns out to be higher than for UV Cet-type stars of equal absolute luminosity.

II Tau could represent a transition prototype between UV Cet-type flare star in the solar neighbourhood and cluster ones.

The importance of coordinated photoelectric patrol of properly selected flare stars in clusters of different age is emphasized.

Flare colours determined from simultaneous UBV observations made at Catania Observatory and from sequential UBV observations made at McDonald Observatory are presented. They fit fairly well with the theoretical colours computed according to the Gurzadian's (1970) non-thermal model. Only part of the observed flare colours are consistent with the solar type models by Gershberg (1967) and Kunkel (1970).

From a B-band patrol of UV Cet-type stars carried out from 1967 to 1972, some quantitative estimates of flare frequencies and luminosities and of their average contribution to the stellar radiation are given.

The corresponding parameters for the Sun, which were estimated from ‘white light’ flare activity, are also given for comparison. The Sun and V 1216 Sgr can be regarded as low-activity flare stars of the type found by Kunkel (1973).

Magnetohydrodynamic considerations on the origin of stellar flares.

This paper reports the researches of the Texas flare star group. High time resolution and simultaneous photometric, spectroscopic and, if possible, radio observations of flaring stars are essential. Fast and slow flares are distinguished and their distinct optical, spectroscopic, and probably, radio effects are noted. The starspot model satisfactorily accounts for many optical and spectroscopic flare star properties, including the precursor phenomenon and quasi periodicity of flaring found by some observers. The basic mechanism of flares is thought to be magnetic field collapse on a scale vastly greater than in the Sun, but further observations are essential, even though there are theoretical grounds for support. No analogue of the solar cycle has yet been detected. The problem of duplicity remains a puzzle.

During several intervals totalling 26 h, 84 flares of UV Ceti were observed using a photometer with 1 s time resolution. Using solar flare terminology, all flares of UV Ceti observed here can be characterized by a typical light curve, consisting of a preflare, a flash and a slow phase. Observations of Bopp and Moffett (1973) show that the flash phase of UV Ceti is dominated by continuous and Hα emission. UV Ceti flare flash phases are compared with solar ‘white-light’ emission during the flash phase and it is suggested that during all UV Ceti flares strong-particle emission occurs, as is the case for solar white-light flares (Švestka, 1970). The characteristics of the solar flare particles are similar to those of the soft cosmic ray sources (Mogro-Campero and Simpson, 1972). If all flare stars emit particles with these characteristics they could produce the soft cosmic ray component in agreement with the work of Comstock (1969).

A great deal of information on the continuous radiation of T Tau- and similar type stars has been acquired. The cause of the ultraviolet and infrared excess emission is discussed. The possibility that the blue continuum may arise from numerous line emissions is considered. Various possibilities for the cause of the light variations are described, as well as the nature of the emission lines. The special features of the FU Ori stars are mentioned.

New groupings of Hα-emission stars connected with young clusters, associations and nebulae are considered to be possible regions of clusterings of irregular (Orion-type) variables. They are listed in Table I and an example, No. 19, is shown in Figure 1.

The light variation in V, B — V, U — B is shown and the change in the Hα, Hβ emission-line profiles is described.

Four T Tau stars and related objects (RY Tau, T Tau, AB Aur and V1057 Cyg) have been included in our spectroscopic programme since 1973. The present paper is concerned with the spectroscopic observations made at the Crimea with the single-stage image tube S1. Tentative atomic line identifications are given for programme stars. CaII and OI emission line equivalent widths and profiles are presented for RY Tau, T Tau and AB Aur. The λ10830 Å line of neutral helium has shown P Cyg-type features for T Tau and V 1057 Cyg.

The photographic magnitude of V 1057 Cyg is estimated on all plates available at the Odessa plate depositary. The light variation is similar to that of FU Orionis.

A general review of the RCB stars is given. The available observations (spectra, photometry (including infrared) and polarization) seem most easily explained by a model in which a cloud of dust is ejected towards the observer at times of deep minima. The large radius changes in the pulsation cycle of RY Sgr suggest a high luminosity. This is also indicated by the RCB stars in the LMC. Theoretical work on RCB stars is briefly reviewed. The possible relation of the variables to the planetary nebulae and the novae is mentioned.

There seems to exist a small subgroup of irregular variables with light curves similar to R CrB stars but with a shorter time scale. These stars have small-amplitude light variations around a ‘normal brightness’, but this is interrupted by Algol-like minima of about to at irregular intervals. The duration of the minimum phase is several percent of the total time. While at minimum the variables also show rapid light variation of greater amplitude. Moreover, RZ Psc shows a flare activity, which strongly resembles that of UV Cet stars. The minimum phase is followed by a rapid recovery to the ‘normal brightness’.