A new analysis of the periods and amplitudes of 12 double-mode RR Lyrae stars in M15 is presented. The results are in excellent agreement with those derived by Cox, Hodson, and Clancy (1983) from the same data using Fourier series techniques. The mean beat mass for 11 stars is 0.658 + or 0.004 (internal standard error) + pr - 0.05 (estimated probable uncertainty). Uncertainties are discussed and light curves are presented for all the stars. Experiments are described that show the effects of random scatter on the derived periods of the RRd stars in M15 and Draco. They provide evidence that the detected secondary oscillations probably are real.
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J. M. Nemec (1985) studied this question.