The Astrophysical Journal, 412:183-191, 1993 July 20


              THE RR LYRAE VARIABLES IN THE GLOBULAR CLUSTER M68

                   CHRISTINE M. CLEMENT AND STEPHEN FERANCE
   David Dunlap Observatory, Department of Astronomy, University of Toronto,
                       Toronto, Ontario, Canada, M5S 1A1

                                      AND

                                NORMAN R. SIMON
Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588

                                   ABSTRACT

       New observations, made with the Helen Sawyer Hogg telescope at Las
    Campanas, have been analyzed in a search for double-mode pulsators (RRd
    stars) in the metal-poor globular cluster, Messier 68. Of the 30 stars
    studied, nine have been identified as RRd stars; V33, which exhibited
    the characteristics of an RRd star in 1950, now appears to be an RRc
    star. Reliable periods and period ratios have been determined for six
    of the RRd stars. Masses for these RRd stars, calculated from fitting
    formulas given by Kovacs, Buchler, & Marom, range from 0.75 to 0.90
    solar masses, depending on the assumed luminosity and metal abundance.
    These masses are in the same range as those for the RRd stars in M15,
    whose RRd sample resembles that of M68 very closely. Fourier parameters
    determined for the light curves of the M68 variables show that the RRc
    stars in the two clusters are also very similar. In particular, on the
    plot of phase parameter Phi_{31} with period, the M15 and M68 RRc
    samples are virtually indistinguishable. A comparison of the new M68
    observations with observations made 40 yr ago shows that the periods of
    some of the stars have changed, but the 40 yr interval is too short for
    detecting period changes caused by evolutionary effects.

    Subject headings:  globular clusters: individual (M68) -- stars:
    variables: other (RR Lyrae) -- stars: oscillations