New SX Phe variables in the globular cluster NGC 288

Abstract

We report the discovery of two new variable stars in the metal-poor globular cluster NGC 288, found by means of time series CCD photometry. We classified the new variables as SX Phoenicis (SX Phe) due to their characteristic fundamental mode periods (1.02 ± 0.01 and 0.69 ± 0.01 h), and refine the period estimates for other six known variables. SX Phe stars are known to follow a well-defined period–luminosity (P–L) relation and, thus, can be used for determining distances; they are more numerous than RR Lyraes in NGC 288. We obtain the P–L relation for the fundamental mode MV = (−2.59 ± 0.18) log P0(d) + (−0.34 ± 0.24) and for the first-overtone mode MV = (−2.59 ± 0.18) log P1(d) + (0.50 ± 0.25). Multichromatic isochrone fits to our UBV colour–magnitude diagrams, based on the Dartmouth Stellar Evolution Database, provide [Fe/H] = −1.3 ± 0.1, E(B − V) = 0.02 ± 0.01 and absolute distance modulus (m − M)0 = 14.72 ± 0.01 for NGC 288.