Deep HST Photometry of NGC 6388: Age and Horizontal-Branch Luminosity

Using the Hubble Space Telescope, we have obtained the first color-magnitude diagram (CMD) to reach the main-sequence turnoff of the Galactic globular cluster NGC 6388. From a comparison between the cluster CMD and that of 47 Tucanae, we find that the bulk of the stars in these two clusters have nearly the same age and chemical composition. On the other hand, our results indicate that the blue horizontal branch and RR Lyrae components in NGC 6388 are intrinsically overluminous, which must be due to one or more, still undetermined, noncanonical second parameter(s) affecting a relatively minor fraction of the stars in NGC 6388.