CT1T2 Photometry of red giants in 2 globulars

By examining the spatial distribution of stellar colors across the red giant branch and using spectroscopic data on several bright giants, we found that NGC 6388's center is affected by differential reddening of at least {DELTA}E(B-V)~0.1. Most of the width of the red giant branch is caused by that differential reddening.

The southeastern outer regions of the cluster seem to be less affected by variable dust extinction, and we find that NGC 6388 is slightly more metal-rich than 47 Tuc, with a distinct asymptotic giant branch. We have put an upper limit on possible variations in [Fe/H] for NGC 6388, finding that it must be less than ~0.2dex. It is unlikely that such a small range in metallicity could be responsible for the blue horizontal branch and blue tail stars in a cluster with a metallicity similar to the more massive 47 Tuc, which only exhibits a red horizontal branch.