Double-Mode RR Lyrae Stars in the Globular Cluster M15

This is the authors' abstract. We don't add key points for this paper.

We have reinvestigated the pulsational behavior of seven previously known or suspected double-mode RR Lyrae stars in the globular cluster MiS using the V and R band photometry of Silbermann & Smith [AJ, 110, 704 (1995a)]. Five of these stars (v17, v30, v31, v39, and v53) continued to exhibit double-mode behavior in the years 1991-1992, while we were unable to confirm double-mode pulsation in vS 1 and v54 during those years. Comparison of our results with those of previous investigations demonstrates that v17, v3 1, and v39 have shown no significant changes in period ratio or amplitude ratio during the past half century. The ratio of first overtone to fundamental mode pulsation in v30 and possibly v53 has changed during the past half century. The evidence that v54 was at one time a double-mode pulsator is doubtful. The evidence for past double-mode behavior of vS 1 is stronger, but still uncertain. For the confirmed double-mode RR Lyrae stars the amplitude of the fundamental mode pulsation can be as large as about half the amplitude of the first overtone pulsation.