1999ApJ...513..767C

The RR Lyrae variable V79 in M3 has been studied on approximately 1000 photographs in the Sternberg Observatory plate collection and on 126 CCD frames. The photographs were obtained over the interval 1949-1992, and the CCD observations were made in 1998. A comparison of our data with observations made in 1992, 1993, and 1996 indicates that V79 changed its pulsation mode in 1992. Before 1992, it was a fundamental mode pulsator (RRab star) with an irregular light curve, but since then it has been a double-mode pulsator (RRd star) with the first overtone the dominant mode. Prior to the mode change, there were sporadic episodes when pulsation in the first overtone mode occurred, for example, for a few days in 1976 and again in 1989. In each case, however, the fundamental mode pulsation resumed afterward. During the first two weeks in April of 1989, the star appeared to stop varying, and when it began again, the period of the fundamental mode pulsation had decreased substantially, from 0.483 to 0.480 days. Although the first overtone has been the dominant mode of pulsation for V79 since 1992 when it became an RRd star, our 1998 observations show that the amplitude of the fundamental mode is greater than it was in 1996. Previously published investigations of the Oosterhoff type II cluster M15 indicate that some of its RRd stars have exhibited irregular behavior, but no mode switches have been observed. In addition, there are some short period RRab stars in other Oosterhoff type I clusters that have changed their light curve amplitude and shape on a timescale of years, but up to now, none of these have been shown to be RRd stars.