Abstract
ABSTRACTThe (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 +28 × 10-6 days day−1 if the pulsation is in the fundamental mode, and +11 × 10-6 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 +16 × 10-6 days day−1; the median value is +14.5 × 10-6 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.
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