1 Intoduction

The Xth IAU General Assembly has defined the UV Cet-type variables as follows: "Dwarf stars of spectral classes dM3e-dM6e characterized by rare and very short flares with amplitudes from 1 mag to 6 mag. Maximum brightness (usually sharp) is attained in a few, or several tens of seconds after the commencement of the flare, total duration of the flare being equal to about ten to fifty minutes''. This pure photometrical definition is kept in all editions of the General Catalogue of Variable Stars.

We propose a more general astrophysical definition: the UV Cet-type variables are stars on the lower part of the main sequence which show phenomena inherent to the solar activity. The most manifestations of the solar activity are detected on such stars: sporadic flares, dark spots, variable emissions from chromospheres and coronae, radio, X-ray and UV bursts. Numerous and well studied manifestations of solar-type activity provide a key to understanding the related stellar variability, while the observations of numerous UV Cet-type stars of different masses and ages, single objects and components of binary systems, provide an opportunity to approach to the solar activity from an evolutionary point of view and to examine its dependence on fundamental stellar parameters.

The UV Cet-type stars are known in the solar vicinity and in the nearest stellar clusters. The number of these objects in such clusters is many hundreds, but information about individual stars is very limited, and they are used mainly for stellar statistical investigations (Haro & Chavira 1965; Ambartsumian & Mirzoyan 1977; Mirzoyan et al. 1990; Caillault 1994). The number of known UV Cet-type stars in the solar vicinity is less by about an order of magnitude, but these objects are accessible, as a rule, for detailed astrophysical studies and give us a physical picture of their activity.

In recent years, a lot of new UV Cet-type stars in the solar vicinity have been discovered by traditional photometry and spectroscopy and by new kinds of researches in the UV and X-ray ranges. In particular, recently Hawley et al. (1996) gave a list of 321 dMe stars with rather strong H-alpha emissions and 3/4 of them should be regarded as new UV Cet-type stars in accordance with our definition. As a result, the existing catalogues of the UV Cet-type stars (Pettersen 1976; Shakhovskaya 1978; Pettersen 1991) turned out to be significantly non-complete. It stimulated us to carry out this work.