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The detached eclipsing binary V20 in the old, metal--rich open cluster NGC6791 is studied in order to determine highly accurate masses and radii of itscomponents. This allows the cluster age to be established with high precision,using isochrones in the mass-radius diagram. We employ high-resolution UVESspectroscopy of V20 to determine the spectroscopic orbit and time-series V, Iphotometry to obtain the photometric elements. The masses and radii of the V20components are found to be1.074+/-0.008Msun and 1.399+/-0.016Rsun (primary) and0.827+/-0.004Msun and 0.768+/-0.006Rsun (secondary). The primary is locatedalmost exactly at the hottest point along the cluster isochrone, and thesecondary is a ~7 times fainter main--sequence star. We determine an apparentcluster distance-modulus of (m-M)_V = 13.46+/-0.10 (average of primary andsecondary). The cluster age is obtained from comparisons with theoreticalisochrones in the mass--radius diagram. Using the isochrones fromVictoria--Regina with [Fe/H] =+0.37 we find 7.7+/-0.5Gyr, whereas theYonsei-Yale (Y2) isochrones lead to 8.2+/-0.5Gyr, and BaSTI isochrones to9.0+/-0.5Gyr. In a mass-radius diagram, the 7.7Gyr VRSS and 9.0Gyr BaSTIisochrones overlap nearly perfectly despite the age-difference. This modeldependence, which is significantly larger than the precision determined frommass, radius, and abundance uncertainties, prevents a definitiveage-determination of the cluster. By observing a suitable number of detachedeclipsing binaries in several open clusters it should be possible to calibratethe age--scale and provide firm constraints which stellar models mustreproduce. (abridged)

A new standard: Age and distance for the open cluster NGC 6791 from the eclipsing binary member V20