SX-PHOENECIS STARS IN THE EXTREMELY METAL-POOR GLOBULAR-CLUSTER NGC-5053

The results of a major search for photometrically variable blue stragg ler stars (BSs) in the extremely metal-poor globular cluster NGC 5053 are presented. The survey is based on photometry of over 200 CCD frame s (BVI passbands) taken on 18 nights between 1985 and 1994. Five of th e 16 BSs monitored for variability are identified as SX Phe stars and their photometric characteristics derived. These five stars are among the shortest-period (49<P<57 min), lowest luminosity (3.0 less than or equal to M(V) less than or equal to 3.3 mag), smallest amplitude (0.0 8 less than or equal to A(V) less than or equal to 0.25 mag) SX Phe st ars presently known. Their colors and locations in both B and V period -luminosity (P-L) diagrams suggest that four of the stars (blue stragg lers 11, 13, 14, and 15 in the Nemec & Cohen 1989 catalog) are fundame ntal-mode pulsators, while the fifth and bluest star, No. 7, appears t o be a first-overtone pulsator. One of the stars, No. 11, exhibits cyc le-to-cycle light variations that are consistent with multimode pulsat ions; however, not enough photometric information is available to deri ve its secondary pulsation period. Masses for the stars, estimated dir ectly from the general equation of stellar pulsation and relative to t he RR Lyrae stars, range from 0.53 M(circle dot) (NC7) to 1.8 M(circle dot) (NC13). The SX Phe stars are used to derive improved P-L-[Fe/H] relations for SX Phe stars, from which a revised distance modulus of ( m-M)(0)=16.06 is obtained for NGC 5053. (C) 1995 American Astronomical Society.