Variable Stars in the Unusual, Metal-rich Globular Cluster NGC 6388

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388, using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388 to ∼57. A significant number of the variables are RR Lyrae (∼14), most of which are probable cluster members. The periods of the fundamental-mode RR Lyrae are shown to be unusually long compared with metal-rich field stars. The existence of these long-period RRab stars suggests that the horizontal branch of NGC 6388 is unusually bright. This implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388 are indeed metal-rich. We consider the alternative possibility that the stars in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids were also found. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain Population II Cepheids. The mean V magnitude of the RR Lyrae is found to be 16.85 ± 0.05, resulting in a distance of 9.0 to 10.3 kpc for a range of assumed values of ⟨MV⟩ for RR Lyrae. We determine the reddening of the cluster to be E(B-V) = 0.40 ± 0.03 mag, with differential reddening across the face of the cluster. We discuss the difficulty in determining the Oosterhoff classification of NGC 6388 and NGC 6441 due to the unusual nature of their RR Lyrae and address evolutionary constraints on a recent suggestion that they are of Oosterhoff type II.