Variable stars in the globular cluster M 92

Results of a search for variable stars in the central region of the Oosterh off II type globular cluster M 92 are presented. Out of the 28 variable and suspected variable stars listed in the Catalogue of Variable Stars in Glob ular Clusters (Clement 1997) only two were not observed, Surprisingly, almo st half of the observed suspected variables did not show ally evidence of v ariability, Only one out of the 11 candidate RR Lyrae variables of Kadla et al. (1983) appeared to ie variable, Moreover, variable v7, until now class ified as: an RE Lyrae star with a period of about 0.515 d, turned out to be of the BL Herculis type, with the period approximately tn ice that long, I n addition, six new variables were found in the very sore of the cluster: f our of the RR Lyrae and the remaining tno of the SX Phoenicis type. RRc var iable v11 shows changes in the light curve shape. DUE to the aliasing probl ems, however, we Call State only that this star is biperiodic. There is a p ossibility that v11 belongs to the new group of recently discovered double- period RRc variables pulsating in non-radial mode, altogether. light curves were obtained for 20 variable stars. Tile total number of known RR Lyrae s tars in h:L 92 equals now 17, 11 of type RRab and the remaining 6, RRc. The mean period of RRab variables amounts to 0.63 +/- 0.05 d, while the averag e period for RRc variables is equal to 0.36 +/- 0.05 d. Mean V magnitudes, mean V - I-C colours, and ranges of variations were derived for 12 RR Lyrae stars and the BL Herculis variable. Period-shift analysis for M 92 and M 2 resulted in finding that despite significant difference in metal abundance between these two clusters ([Fe/H] = -2.24 for h:I 92 and -1.62 for M 2), there is no firm evidence for a shift in period.