Variable Stars in Metal-Rich Globular Clusters. III. NGC 6539

We present the first CCD-based search for variable stars in the metal-rich globular cluster NGC 6539 ([Fe/H] ≈ -0.7). Our VI photometry of stars within 7′ of the cluster center shows a color-magnitude diagram with a normal red clump horizontal-branch morphology without the extension of blue stars seen in two globular clusters of comparable metallicity, NGC 6388 and NGC 6441. We have discovered 13 variable stars, 12 of which are long-period variables (LPVs), and have used infrared photometry from the Two Micron All Sky Survey to further clarify their position in the color-magnitude diagram. We find the specific frequency of these stars, SLPV = 0.32, to be comparable to those found in other clusters recently surveyed. We have also detected one RR Lyrae variable star in our data, although it is unlikely to be a cluster member. The color of this star, when compared with low-reddening RR Lyrae stars near the Sun, indicates an interstellar reddening of E(V - I) = 1.22 mag toward NGC 6539, in excellent agreement with estimates from the literature.