Fourier analysis of the light curves of the RR Lyrae stars in the globular clusters M 15, NGC 6171, and NGC 6723

Abstract

We compare the Fourier decomposition parameters of the B light curves of the RR Lyrae variables in the globular clusters NGC 6171, NGC 6723 (Osterhoff I), and M15 (Osterhoff II). Indications are that there may be small systematic differences between the Fourier coefficients of the variables of the two Osterhoff classes. The progression of the Fourier coefficients and their interrelations are very similar to (although less pronounced than) those in ω Cen. The significance and possible implications of these progressions on stellar parameters are briefly discussed. The first overtone variables are well separated from the fundamental ones in all parameter relations. A least-squares frequency analysis of those stars, which show greater scatter in the phase diagrams than the observational errors allow, reveals the complexity of their light variations and points to further double-mode suspects in each cluster. Analysis of the two separate data sets of M15, which were taken 20 yr apart, shows that, except perhaps for a few cases, there is no significant change in the light variations between the two epochs. The known double-mode RR Lyrae stars with reliable period identifications have not changed their modal contents either.