The RR Lyrae Stars

For more than fifty years the RR Lyrae stars have played a prominent role in problems of galactic structure, first as distance indicators and more recently as population indicators. This has followed from the apparent homogeneity of the group, the large number present in the Galaxy, and their moderately high luminosities, which render them visible at large distances. However, in the past decade it has become clear that RR Lyrae stars are heterogeneous in many observable parameters. This heterogeneity limits their usefulness in some contexts and enhances their value in others. In addition to their applications in galactic research, the RR Lyrae stars are of current astrophysical interest on account of the line-doubling and emission that occur during rising light, the multiple periodicities found in many of them, and various other phase- and period-dependent phenomena that bear on the theory of stellar pulsation. In this review we summarize recent developments in these areas.