vbyCaHβ CCD Photometry of Clusters. VIII. The Super-Metal-Rich, Old Open Cluster NGC 6791

CCD photometry on the intermediate-band vbyCaHβ system is presented for the metal-rich, old open cluster NGC 6791. Preliminary analysis led to [Fe/H] above +0.4 with an anomalously high reddening and an age below 5 Gyr. A revised calibration between (b - y)₀ and [Fe/H] at a given temperature shows that the traditional color-metallicity relations underestimate the color of the turnoff stars at high metallicity. With the revised relation, the metallicity from hk and the reddening for NGC 6791 become [Fe/H] = +0.45 ± 0.04 and E(b - y) = 0.113 ± 0.012 or E(B - V) = 0.155 ± 0.016. Using the same technique, reanalysis of the photometry for NGC 6253 produces [Fe/H] = +0.58 ± 0.04 and E(b - y) = 0.120 ± 0.018 or E(B - V) = 0.160 ± 0.025. The errors quoted include both the internal and external errors. For NGC 6791, the metallicity from m₁ is a factor of 2 below that from hk, a result that may be coupled to the consistently low metal abundance from DDO photometry of the cluster and the C-deficiency found from high-dispersion spectroscopy. E(B - V) is the same value predicted from Galactic reddening maps. With E(B - V) = 0.15 and [Fe/H] = +0.45, the available isochrones predict an age of 7.0 ± 1.0 Gyr and an apparent modulus of (m - M) = 13.60 ± 0.15, with the dominant source of the uncertainty arising from inconsistencies among the isochrones. The reanalysis of NGC 6253 with the revised lower reddening confirms that on both the hk and m₁ metallicity scales, NGC 6253, while less than half the age of NGC 6791, remains at least as metal-rich as NGC 6791, if not richer.