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Description
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 (1996, Cat. <J/A+A/312/111>) 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.
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