Wide-field Variability Survey of the Globular Cluster M79 and a New Period -- Luminosity Relation for SX Phe Stars

We present the results of a search for variable stars in a 26x39 arcmin^2 field around globular cluster M79 (NGC1904). The search was made by means of an extended version of image subtraction, which allows to analyze in a uniform manner CCD frames obtained with different telescopes and cameras of different sizes and resolutions. The search resulted in finding 20 new variable stars, among which 13 are cluster members. The members include one new RR Lyr star of subtype c, three SX Phe stars, and nine variable red giants. We also show that V7 is a W Vir star with a period of 13.985 d. Revised mean periods of RRab and RRc stars, =0.71 d and =0.34 d, respectively, and relative percentage of RRc stars, N_c/(N_ab+N_c)=45 % confirm that M79 belongs to the Oosterhoff II group of globular clusters. The mean V magnitude of the horizontal branch of M79 based on ten RR Lyr stars has been estimated to be $V_HB= =16.11\pm0.03$ mag. In one RRc star, V9, light changes with three close frequencies were detected, indicating excitation of nonradial modes. An SX Phe star, V18, is a double-mode pulsator with two radial modes excited, fundamental and first overtone. Moreover, we have discovered two SX Phe or delta Sct stars and one W UMa type system, all likely field objects. We also studied the period -- luminosity relation for SX Phe stars. Using 62 fundamental and fundamentalized periods of radial double-mode and high-amplitude SX Phe stars known in Galactic globular clusters, we have derived the slope and zero point of this relation to be, $-3.35\pm0.24$ and $2.68\pm0.03$ mag (at log(P/d)=-1.24), respectively.