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Mira variables, RR Lyrae variables, and type II Cepheids all representevolved states of low-mass stars, and long term observations have revealed thatchanges in pulsation period occur for each of these classes of variable. MostMira variables show small or no period changes, but a few show large periodchanges that can plausibly be associated with thermal pulses on the asymptoticred giant branch. Individual RR Lyrae stars show period changes that do notaccord with the predictions of stellar evolution theory. This may be especiallytrue for RR Lyrae stars that exhibit the Blazhko effect. However, the averageperiod changes of all of the RR Lyrae variables within a globular cluster provea better but still imperfect match for the predictions of evolutionary theory.The observed period changes of short period type II Cepheids (BL Her stars) aswell as those of long period type II Cepheids (W Vir stars) are in broadagreement with the rates of period changes expected from their evolutionarymotions through the instability strip.

Period Changes of Mira Variables, RR Lyrae Stars, and Type II Cepheids