Context : We wish to determine accurate ages for open clusters and use this , in conjunction with colour-magnitude diagrams , to constrain models of stellar structure and evolution .

Aims : The detached eclipsing binary V20 in the old , metal–rich ( [ \mathrm { Fe / H } ] = +0.40 ) open cluster NGC 6791 is studied in order to determine highly accurate masses and radii of its components .
This allows the cluster age to be established with high precision , using isochrones in the mass-radius diagram .

Methods : We employ high-resolution UVES spectroscopy of V20 to determine the spectroscopic orbit and time-series V,I photometry to obtain the photometric elements .

Results : The masses and radii of the V20 components are found to be 1.074 \pm 0.008 M _ { \sun } and 1.399 \pm 0.016 R _ { \sun } ( primary ) and 0.827 \pm 0.004 M _ { \sun } and 0.768 \pm 0.006 R _ { \sun } ( secondary ) .
The primary is located almost exactly at the hottest point along the cluster isochrone , and the secondary is a \sim 7 times fainter main–sequence star .
We determine an apparent cluster distance-modulus of ( m - M ) _ { V } = 13.46 \pm 0.10 ( average of primary and secondary ) .
The cluster age is obtained from comparisons with theoretical isochrones in the mass–radius diagram .
Using the isochrones from Victoria–Regina with [ \mathrm { Fe / H } ] = +0.37 we find 7.7 \pm 0.5 Gyr , whereas the Yonsei-Yale ( Y ^ { 2 } ) isochrones lead to 8.2 \pm 0.5 Gyr , and BaSTI isochrones to 9.0 \pm 0.5 Gyr .
In a mass-radius diagram , the 7.7Gyr VRSS and 9.0Gyr BaSTI isochrones overlap nearly perfectly despite the age-difference .
This model dependence , which is significantly larger than the precision determined from mass , radius , and abundance uncertainties , prevents a definitive age-determination of the cluster .

Conclusions : Using detached eclipsing binaries for determination of cluster ages , the dominant error is due to differences among stellar models and no longer to observational errors in cluster reddening and distance .
By observing a suitable number of detached eclipsing binaries in several open clusters it should be possible to calibrate the age–scale and provide firm constraints which stellar models must reproduce .