TEMPORAL BEHAVIOR OF SOURCES IN THE ROSAT EXTREME-ULTRAVIOLET ALL-SKYSURVEY

During the ROSAT all-sky survey (1990 July - 1991 January), each sourc e detected by the extreme-ultraviolet (EUV) Wide Field Camera (WFC) wa s observed once per approximate to 1.5 h, for typically five days, dic tated by the characteristics of the scanning survey. Light-curves for each of the 479 sources in the WFC 2RE Catalogue have been generated a nd tested for constancy on time-scales similar to 1.5 h. As a result, 31 sources have been classified as variable. The main constituents of the 2RE Catalogue are, in order, Ide-type stars (F-M), hot white-dwarf stars, and cataclysmic variables. CVs are found to be the most active group with approximate to 50 per cent of them displaying variability, compared with approximate to 10 per cent of late-type stars. All the variable G-type stars are known or possible RS CVn-type binaries; simi lar to 40 per cent of the 2RE G-type RS CVn systems are found to vary. Other variable sources are the high-mass X-ray binary Her X-1, the ec lipsing binaries Algol (B8 V + K0 IV) and V471 Tau (DA + K2 V), and th e active galaxy Mrk 478.