Composition and Evolution Patterns in the Old Open Cluster Melotte 66: Photoelectric (U) BV(RI) Photometry of 71 Stars

New photoelectric (U) BV(RI) data for 71 red giants and turnoff stars in the old open cluster Melotte 66 are presented and discussed. The photometry confirms the broad color range exhibited by giants at and above the level of the clump. UBV analysis of turnoff stars yields [Fe/H]= — 0.53 ± 0.08 assuming a cluster E(B— V) = 0.16 ± 0.02. A nearly identical [Fe/H] is implied by the giants' ultraviolet excesses, but with a significantly higher dispersion. Previously published CCD vby data have been transformed to the BV system to permit an analysis of the cluster color-magnitude diagram with reference to isochrones which incorporate convective core overshoot. Main-sequence fits imply an age of 4.5 ± 0.5 Gyr and an apparent modulus (m — M)= 13.75. Further support for these reddening and distance values is provided by comparison of the red-giant clump to other old open clusters and of the M/,(V— I) data to a sample of globular and old open clusters. The giant branch of Mel 66 is shown to extend to an extreme in both luminosity and color currently matched only by old open clusters NGC 6791 and Cr 261.