  An extensive grid of metal-rich isochrones utilizing the latest available
input physics has been calculated for comparison with the old, metal-rich open
cluster NGC 6791. The isochrones have been simultaneously fit to BV and VI
color magnitude diagrams, with the same composition, reddening and distance
modulus required for both colors. Our best fitting isochrone assumes [Fe/H] =
+0.4, scaled solar abundance ratios, and dY/dZ = 2 (Y = 0.31), yielding an
excellent fit to the data at all points along the major 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 the isochrone [Fe/H] and
helium abundances. All of the acceptable fits indicate that 0.07 < E(B-V) <
0.14$, 13.29 < (m-M)_v < 13.46, and that NGC 6791 has an age of 8.0+/- 0.5 Gyr.
The fits also suggest that dY/dZ lies between 1 and 3. A metallicity as low as
solar is clearly ruled out, as is dY/dZ = 0. Comparison with previous isochrone
studies indicates that the derived reddening is primarily due to our use of the
most recent color transformations, whereas the age depends upon both the colors
and 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.
