THE ASTRONOMICAL JOURNAL    VOLUME 110, NUMBER 3, PAGE 1186      SEPTEMBER 1995

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

                                JAMES M. NEMEC

   Department of Physics and Astronomy, York University, 4700 Keele Street,
                      North York, Ontario M3J 1P3, Canada
                       Electronic mail: nemec@dao.nrc.ca

                                  MARIO MATEO

               Department of Astronomy, University of Michigan,
                        Ann Arbor, Michigan 48109-1090
                  Electronic mail: mateo@astro.lsa.umich.edu

                                 MORGAN BURKE

              TRIUMF, University of British Columbia, Vancouver,
                       British Columbia V6T 1Z4, Canada

                              EDWARD W. OLSZEWSKI

       Steward Observatory, University of Arizona, Tucson, Arizona 85721
                       Electronic mail: ewo@steward.edu


                                   ABSTRACT

    The results of a major search for photometrically variable blue
    straggler stars (BSs) in the extremely metal-poor globular cluster NGC
    5053 are presented.  The survey is based on photometry of over 200 CCD
    frames (BVI passbands) taken on 18 nights between 1985 and 1994.  Five
    of the 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 <~
    M_V <~ 3.3 mag), smallest amplitude (0.08 <~ A_V <~ 0.25 mag) SX Phe
    stars presently known.  Their colors and locations in both B and V
    period-luminosity (P-L) diagrams suggest that four of the stars (blue
    stragglers 11, 13, 14, and 15 in the Nemec & Cohen 1989 catalog) are
    fundamental-mode pulsators, while the fifth and bluest star, No. 7,
    appears to be a first-overtone pulsator.  One of the stars, No. 11,
    exhibits cycle-to-cycle light variations that are consistent with
    multimode pulsations; however, not enough photometric information is
    available to derive its secondary pulsation period.  Masses for the
    stars, estimated directly from the general equation of stellar
    pulsation and relative to the RR Lyrae stars, range from 0.53 Msun
    (NC7) to 1.8 Msun (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.