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 \hbox { $ { [ Fe / H ] } $ } = +0.4 , scaled solar abundance ratios , and \hbox { $ \Delta Y / \Delta Z$ } = 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 \hbox { $ { E ( B - V ) } $ } = 0.10 and \hbox { $ ( 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 0.08 \leq \hbox { $ { E ( B - V ) } $ } \leq 0.13 , 13.30 \leq \hbox { $ ( m - M ) _ { V } $ } \leq 13.45 , and that NGC 6791 has an age of 8.0 \pm 0.5 Gyr .
The fits also suggest that \Delta Y / \Delta Z lies between 1 and 3 .
A metallicity as low as solar is clearly ruled out , as is \hbox { $ \Delta Y / \Delta Z$ } = 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 .