Searching for Variable Stars in the Central Part of the Globular Cluster M22

Time-series data taken with the Hubble Space Telescope of three fields covering the central part of the globular cluster M22 have been analyzed in search of variable objects. We report identification of eleven periodic variables of which eight are new discoveries. The sample includes five certain contact binaries as well as one SX Phe star. Two objects with periods longer than 1 day are preliminarily classified as either spotted variables of BY Dra type or ellipsoidal variables. The most unusual of the identified variables, M22_V11, has IC≈19.9 and is located far to the red of the main sequence in the cluster color-magnitude diagram. It shows variability with a period as short as P≈0.066 days or alternatively with P≈0.132 days. We propose that it may be an ellipsoidal variable harboring a degenerate component.