THE ASTRONOMICAL JOURNAL    VOLUME 110, NUMBER 2, PAGE 704          AUGUST 1995

            THE RR LYRAE VARIABLE STARS IN THE GLOBULAR CLUSTER M15

                     N. A. SILBERMANN AND HORACE A. SMITH

        Department of Physics and Astronomy, Michigan State University,
                         East Lansing, Michigan 48824
       Electronic mail: nancys@ipac.caltech.edu, smith@msupa.pa.msu.edu


                                   ABSTRACT

    New CCD photometry is presented for RR Lyrae variable stars in the
    globular cluster M15. This photometry, mainly in V and R, with some
    additional B and I, is used to construct lightcurves of 44 RR Lyraes
    and one Cepheid in M15.  One new low-amplitude RR Lyrae star was
    discovered.  The observable parameters of the light curves are used, in
    conjunction with theoretical photometric indices, to obtain effective
    temperatures and bolometric magnitudes in the RR Lyrae stars.  We
    confirm that the regions occupied by RRab and RRcd variables in M15
    have little or no overlap in effective temperature.  Photometry for
    variable and nonvariable horizontal branch stars in M15 is used to
    establish the high and low temperature boundaries of the instability
    strip.  There is evidence that the blue edge of the M15 RR Lyrae
    instability strip becomes cooler at higher luminosities.  Employing
    Cox's value of the mass of RRd stars, 0.75 Msun, and the pulsation
    equation, the absolute magnitude of the RR Lyrae stars in M15 is
    estimated to be <M_V> = +0.36 +/- 0.12 mag.  However, this result
    depends upon the adopted mass, bolometric correction, and
    color-temperature relation, which remain significantly uncertain.  The
    implications of the absolute magnitude determination for the ages of
    the globular clusters are discussed.