1999PASP..111...98P

The (O-C) diagrams of Mira stars are dominated by the effects of random, cycle-to-cycle fluctuations in period. Nevertheless, it is possible to average these effects over many stars with similar properties and compare the average apparent period change with that predicted by evolutionary models. The average rate of period change for a 1.0 M_☉ model, averaged over the Mira phase, is +28x10^–6 days.day^–1 if the pulsation is in the fundamental mode, and +11x10^–6 days.day^–1 if it is in the first overtone mode (as most Mira star pulsation is now believed to be). The observed changes are not inconsistent with these results, in the sense that (1) positive period changes outnumber negative ones and (2) the average period change of all of the M type Miras is +16x10^–6 days.day^–1; the median value is +14.5x10^–6 days.day^–1. Of the seven M type Miras with [V-12 µm] colors greater than 13.5, six had positive period changes. There is some evidence that C type Miras have larger period changes than M and S types.