A LARGE X-RAY FLARE ON HU VIRGINIS

We present two ROSAT-HRI observations of the RS CVn-type binary HU Vir ginis (=HD 106225). The first observation covers almost three consecut ive rotation cycles of HU Virginis or a total of 26.7 days and an expo sure time of 35 ksec. On JD 2,449,544 a long duration flare was detect ed and observed for 1.5 days. This event was releasing a total energy of approximate to 7.7(-1.9)(+3.3) x 10(36) erg in the 0.1-2.4 keV band pass. The good coverage of the onset and maximum phase of the flare li ght curve allowed a detailed comparison with two solar flare models. W e derived an estimate for the size of the active region responsible fo r the flare. The resulting loop size is of the order of one stellar ra dius. One year later, in 1995, HU Virginis was again observed by ROSAT continuously for 8 days and a total exposure time of 69 ksec. The X-r ay flux shows variability on time scales shorter than the rotational p eriod.