THE ASTRONOMICAL JOURNAL    VOLUME 113, NUMBER 5, PAGE 1711          MAY 1997

          THE STRUCTURE OF THE LIGHT CURVES OF THE RR LYRAE VARIABLES
                   IN THE OOSTERHOFF TYPE I CLUSTER NGC 6171

                     CHRISTINE M. CLEMENT AND IAN SHELTON
                Department of Astronomy, University of Toronto,
                       Toronto, Ontario, M5S 3H8, Canada
    Electronic mail: cclement@astro.utoronto.ca, shelton@astro.utoronto.edu

                                   ABSTRACT

     CCD observations have been used to derive precise light curves and
     Fourier parameters for sixteen RR Lyrae variables in the Oosterhoff
     type I globular cluster NGC 6171. It is found that, for the RRab
     stars, the Fourier phase differences Phi_21, Phi_31 and Phi_41 all
     correlate with period, but the amplitude ratios R_21, R_31, and R_41
     do not. For the RRc stars, R_21, Phi_31, and Phi_41 correlate with
     period, but Phi_21, R_31, and R_41 do not. It is also found that a
     plot of R_21 against Phi_21 is a useful tool for distinguishing
     between RRc and RRab stars, regardless of color or chemical
     composition. Simon's method that relates physical properties of RRc
     stars to the pulsation period and Fourier phase parameter Phi_31
     has been used to determine the mass, luminosity, temperature and a
     helium parameter for the RRc stars. A comparison with the parameters
     determined from CCD observations of RRc stars in six other galactic
     globular clusters indicates that mean masses and mean luminosities
     increase and mean temperatures and helium abundances decrease with
     decreasing cluster metal abundance. The formulae of Jurcsik & Kovacs
     for relating Fourier parameters of RRab stars to [Fe/H] and luminosity
     have been tested and the results are reasonable. (c) 1997 American
     Astronomical Society.