Springer Cosmic Chemical Evolution: Proceedings of the 187th Symposium of the International Astronomical Union, Held at Kyoto, Japan, 26–30 August 1997: 187 (International Astronomical Union Symposia, 187)

The open cluster NGC 6791 is now considered both the oldest and the most metal-rich known. Its age is 8 -10 Gyrs, twice as old as the canonical solar-metallicity cluster M67 (Garnavich et al. 1994; Demarque, Green, & Guenther 1992; Tripicco et al. 1995). That its metallicity is significantly above solar is suggested from moderate-resolution spectroscopy and from a mismatch of its color-magnitude diagram (CMD) with solar-metallicity isochrones. Tripicco et al. (1995) find [Fe/H] = +0.27 to +0.44. The cluster population is rich. In addition to about a dozen red giants and two dozen red horizontal-branch stars, the cluster has several very hot HB stars (Kaluzny & Udalski 1992). Liebert et al. (1994) have shown that the extremely blue stars are mostly sdB/sdO stars and at least 3 or 4 are likely cluster members, the first ever discovered in an open cluster. These may provide the key to the puzzling upturn in ultraviolet flux below 1500A seen in many high-metallicity systems (Burstein et al. 1988; Ferguson et al. & Liebert 1993).