The age, extinction, and distance of the old, metal-rich open cluster NGC 6791

An extensive grid of metal-rich isochrones utilizing the latest available i nput physics has been calculated for comparison with the old, metal-rich op en cluster NGC 6791. The isochrones have been simultaneously fitted to BV a nd VI color-magnitude diagrams, with the same composition, reddening, and d istance modulus required for both colors. Our best-fitting isochrone assume s [Fe/H] = +0.4, scaled solar abundance ratios, and Delta Y/Delta Z = 2 (Y = 0.31), yielding an excellent fit to the data at all points along the majo r sequences. The resulting age is 8 Gyr, with E(B-V) = 0.10 and (m-M)(V) = 13.42. The derived cluster parameters are fairly robust to variations in th e isochrone [Fe/H] and helium abundances. All the acceptable fits indicate that 0.08 less than or equal to E(B-V) less than or equal to 0.13 and 13.30 less than or equal to (m - M)(V) less than or equal to 13.45 and that NGC 6791 has an age of 8.0 +/- 0.5 Gyr. The fits also suggest that Delta Y/Delt a Z lies between 1 and 3. A metallicity as low as solar is clearly ruled ou t, as is Delta Y/Delta Z = 0. Comparison with previous isochrone studies in dicates that the derived reddening is primarily due to our use of the most recent color transformations, whereas the age depends on both the colors an d the input physics. Our isochrones provide an excellent fit to the Hyades zero-age main sequence as determined by Hipparcos, providing evidence that our derived reddening and distance modulus are reliable.