A MAJOR OPTICAL FLARE ON THE RECENTLY DISCOVERED X-RAY ACTIVE DME STAR G-102-21

During the course of UBV photoelectric measurements made at the Catani a Astrophysical Observatory we have observed an unusually intense opti cal flare on the nearby M dwarf G 102-21, the optical counterpart of a strong X-ray source recently detected by ROSAT. The amplitude of the magnitude increase (Delta U similar to 7.3 mag) and the total energy r elease in the UBV bands (E(UBV) similar to (1.4 +/- 0.3) x 10(35) erg) put this event among the strongest flares ever detected on UV Ceti-ty pe stars. The U-B and B-V colours of the flare emission did not signif icantly change during most of the flare development. While the B-V is quite a typical colour for stellar flares, the U-B is much bluer when compared with typical values given in literature. This unusually stron g ultraviolet component of the flare emission, together with the very fast flare decay, suggest a possible flare site close to the limb. We find that the observed amount of flare energy could be explained by th e fast release of energy stored in a magnetic filament located in betw een the two G 102-21 stellar components, with a surface magnetic field greater than or equal to 3600 G. The characteristics of the quiescent optical emission are also discussed.