Variable stars in the globular cluster M 28 (NGC 6626)

Context. We present a new search for variable stars in the Galactic globular cluster M 28 (NGC 6626). Aims. The search is based on a series of BVI images obtained with the SMARTS Consortium's 1.3 m telescope at Cerro Tololo Inter-American Observatory, Chile. Methods. The search was carried out using the ISIS v2.2 image subtraction package. Results. We find a total of 25 variable stars in the field of the cluster, nine being new discoveries. Of the newly found variables, one is an ab-type RR Lyrae star, six are c-type RR Lyrae, and two are long-period/semi-regular variables. V22, previously classified as a type II Cepheid, appears as a bona-fide RRc in our data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be properly phased with any reasonable period. Conclusions. The properties of the ab-type RR Lyrae stars in M 28 appear most consistent with an Oosterhoff-intermediate classification, which is unusual for bona-fide Galactic globulars clusters. However, the cluster's c-type variables do not clearly support such an Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another possibility, thus raising the intriguing possibility of multiple populations being present in M 28. Coordinates, periods, and light curves in differential fluxes are provided for all the detected variables.