OBSERVATION OF POSSIBLE MODE SWITCHING IN 3 SEMIREGULAR VARIABLE-STARS

Both the amplitude and period of the light variations of the semiregular long-period variables RV and, S Aql, and U Boo dramatically decreased, and then suddenly increased, in recent years. These observations, in conjunction with spectroscopic data, suggest that these stars may have switched from fundamental to overtone pulsation and back again. Theoretical models are consistent with both the photometric and spectroscopic observations. The similarity between the longer-period oscillations of these stars and the behavior of Mira variables supports the conclusion that the Miras are fundamental-mode pulsators.