Long-term changes in Mira stars. III. Multiperiodicity in Mira stars

The maximum magnitudes of many Mira stars fluctuate from cycle to cycle. In some cases, this may be due to multiperiodicity. We have analyzed the maximum magnitudes of a large sample of bright Mira stars, using a portion of the American Association of Variable Star Observers international database of variable star observations: times and magnitudes of maximum and minimum of 391 bright Mira stars over 75 years. We find many whose behavior can be interpreted as due to multiperiodicity, with either a secondary period that is an order of magnitude longer than the primary period or a secondary period comparable with the primary one. In the former case, the nature of the long secondary period is not clear, although, in some cases, it may be related to episodic dust emission. In the latter case, the behavior is most consistent with the hypothesis that the primary period is the first overtone period and the secondary period is another radial mode.