We analyzed high-precision Kepler photometry and high-resolution UVES and GIRAFFE spectroscopy from ESO Science Archieve for a double-lined eclipsing binary star in the field of the high metallicity old open cluster NGC 6791. Earlier measurements of the masses and radii of the detached system were not accurate enough for photometric and spectroscopic data to demonstrate that there are significant differences between current stellar models. Here we improved on the result and add follow-up measurements of the system. Data from the Kepler archive makes it clear that the system has an inclination that is close to 90 degrees. The combination of radial velocity and Kepler light curve of the system were analysed simultaneously, which allows us to determine a reliable mass for the primary and secondary star and radii for both stars, and to constrain the cluster age. The characteristics of the primary star at the cluster turnoff indicate an age of 8.5 +/- 0.12 Gyr, consistent with earlier analysis of the color magnitude diagram. The brighter star in the binary also produces a precision estimate of the distance modulus, independent of reddening estimates: (m-M)(V) (mag) =13.899 +/- 0.117. The secondary star is not expected to have evolved significantly, but its radius is more than 10% larger than predicted by models. The hallmark is useful for testing the idea that radius inflation can occur in short period binaries for stars with significant convective envelopes due to the inhibition of energy transport by magnetic fields.