New SX Phoenicis Stars in the Globular Cluster M53

Through time-series CCD photometry of the metal-poor globular cluster M53, we have discovered eight new SX Phoenicis type stars (labeled from SXP1 to SXP8). All the new SX Phoenicis stars are located in the blue straggler star region of a color-magnitude diagram of M53. One of these stars (SXP2) is found to have very closely separated pulsation frequencies: $f_1\mathrm{/}{f}_2=0.9595$ where $f_1$ and $f_2$ are primary and secondary frequencies. This may be due to excitation of non-radial modes. Six of these SX Phoenicis stars are considered to be pulsating in the fundamental mode. They show a tight linear correlation between the period and luminosity. We derive a period - luminosity relation for the fundamental mode for the period range of $-1.36<LogP[d]<-1.15$ : $<V>=-3.010(\pm 0.262)LogP+15.310(\pm 0.048)$ with an rms scatter of 0.038, corresponding to $<M_V>=-3.010LogP-1.070$ for an adopted distance modulus of $(m-M{)}_V=16.38$ (Harris 1996).