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Session 96 - Cepheids and Supergiant Variable Stars.
Display session, Thursday, January 16
Metropolitan Ballroom,

[96.06] Long-Term Changes in Mira Stars

J. R. Percy, W. Au, D. Bagby (UToronto)

This paper reports the results of two studies, carried out using a portion of the American Association of Variable Star Observers (AAVSO) international database of variable star observations: times and magnitudes of maximum and minimum magnitude of 391 bright Mira stars over 75 years (JAAVSO 15, 236 (1986)).

(i) 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 derived period changes with those predicted by evolutionary models (Vassiliadis and Wood, 1993). The observed changes are consistent with the models.

(ii) The maximum magnitudes of many Mira stars fluctuate from cycle to cycle. This may be due to multiperiodicity. We have analyzed the maximum magnitudes of the 391 Mira stars in the sample. We find many whose behaviour can be interpreted as due to multiperiodicity, with either a secondary period an order of magnitude longer than the primary one, or a secondary period comparable with the primary one. In the latter case, the behaviour is most consistent with the hypothesis that the primary period is the first overtone.

Acknowledgement.\/ We thank the AAVSO and its observers for making these studies possible.

Program listing for Thursday