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Description
Results 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 M92 equals now 17, 11 of type RRab and the remaining 6, RRc. The CCD observations presented here were carried out at the Bialkow station of the Wroclaw University Observatory with the same equipment as that described by Kopacki & Pigulski (1995AcA....45..753K). Two 6x4arcmin^2^ fields of view, one centred approximately on the core of M92 and the other shifted about 2.5arcmin north and 2.6arcmin east, were observed through V and I_c_ filters of the Johnson-Kron-Cousins UBV(RI)_c_ system. The observations were carried out in two seasons: on 9 nights between 1998 July 20 and September 1, and on 17 nights between 2000 May 4 and June 20. During the 1998 season only a few frames of the second field were taken. The main field, centred on the core of the cluster, was observed during both seasons.
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