PERIOD CHANGES OF RR LYRAE STARS IN THE GLOBULAR-CLUSTER M15

We have investigated the period changes of 49 RR Lyrae variable stars in the globular cluster M15 using observations spanning almost a century. Some of the variables have been too sparsely observed over that time interval for reliable period changes to be determined. Others, which have been well observed, show a wide range of period change behavior. We find a significant excess of increasing over decreasing periods. The median rate of period change, +0.03 days/million years, agrees with the theoretical prediction of Lee (1991). This implies that most M15 RR Lyrae stars are evolving from blue to red through the instability strip. However, it is not certain whether there are sufficient blue horizontal branch stars in M15 to sustain the M15 RR Lyrae population at the observed rate of period change. The dispersion in the rate of period change for RR Lyraes of Bailey type ab is greater than for those of type cd. The origin of that difference in period change behavior is without adequate explanation.