Color‐Color Relations for Red Giants in Star Clusters

New Johnson-Cousins UBVRI photometry of giants in globular clusters iscombined with JHK photometry on the CIT system to produce color sequences forgiants from the globular clusters M3, M5, M13, and M92. UBVRI data are alsopresented for giants in the metal-rich open cluster NGC 6791. These data fill agap in the literature, especially for the R & I bands. We provide the empiricalrelations between broad band colors for various [Fe/H] values for metal-poorgiants. The color sequences for U-B and B-V show clear separations fordifferent [Fe/H] values. We also find weak, though unexpected, metallicitydependences of V-R, V-I, and J-K colors. H-K is metal-insensitive. The abovecolors are plotted as a function of V-K, and a literature (V-K) - T_{eff}relation is given.Comment: 21 pages, 7 figures; AASTeX 4.0; to appear in the July 1998 edition of PASP. Revised version; includes discussion on possible effects of reddening error