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 due to their characteristic fundamental mode periods ( 1.02 \pm 0.01 and 0.69 \pm 0.01 hours ) , 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 M _ { V } = ( -2.59 \pm 0.18 ) \log P _ { 0 } ( d ) + ( -0.34 \pm 0.24 ) and for the first-overtone mode M _ { V } = ( -2.59 \pm 0.18 ) \log P _ { 1 } ( d ) + ( 0.50 \pm 0.25 ) .
Multi-chromatic isochrone fits to our UBV color-magnitude diagrams , based on the Dartmouth Stellar Evolution Database , provide \langle [ Fe/H ] \rangle = -1.3 \pm 0.1 , E ( B-V ) = 0.02 \pm 0.01 and absolute distance modulus ( m-M ) _ { 0 } = 14.72 \pm 0.01 for NGC 288 .