2001A&A...369..862K

Result. of a search for variable stars in the central region of the Oosterhoff 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 Globular Clusters (Clement, 1997, Cat. V/97) only two were not observed. Surprisingly, almost half of the observed suspected variables did not show any evidence of variability. Only one out of the 11 candidate RR Lyrae variables of Kadla et al. (1983PZ.....21..827K) appeared to be variable. Moreover, variable v7, until now classified as an RR Lyrae star with a period of about 0.515d, turned out to be of the BL Herculis type, with the period approximately twice that long. In addition, six new variables were found in the very core of the cluster: four of the RR Lyrae and the remaining two of the SX Phoenicis type. RRc variable v11 shows changes in the light curve shape. Due to the aliasing problems, however, we can state only that this star is biperiodic. There is a possibility 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. The total number of known RR Lyrae stars in M 92 equals now 17, 11 of type RRab and the remaining 6, RRc. The mean period of RRab variables amounts to 0.63±0.05d, while the average period for RRc variables is equal to 0.36±0.05d. 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 M 92 and -1.62 for M 2), there is no firm evidence for a shift in period.