We present the Kepler photometric light-variation analysis of the late-type double-lined binary system V568 Lyr that is in the field of the high metallicity old open cluster NGC 6791 .
The radial velocity and the high-quality short-cadence light curve of the system are analysed simultaneously .
The masses , radii and luminosities of the component stars are M _ { 1 } = 1.0886 \pm 0.0031 M { \odot } , M _ { 2 } = 0.8292 \pm 0.0026 M { \odot } , R _ { 1 } = 1.4203 \pm 0.0058 R { \odot } , R _ { 2 } = 0.7997 \pm 0.0015 R { \odot } , L _ { 1 } = 1.85 \pm 0.15 L { \odot } , L _ { 2 } = 0.292 \pm 0.018 L { \odot } and their separation is a = 31.060 \pm 0.002 R { \odot } .
The distance to NGC 6791 is determined to be 4.260 \pm 0.290 kpc by analysis of this binary system .
We fit the components of this well-detached binary system with evolution models made with the Cambridge stars and ev ( twin ) codes to test low-mass binary star evolution .
We find a good fit with a metallicity of Z = 0.04 and an age of 7.704 Gyr .
The standard tidal dissipation , included in ev ( twin ) is insufficient to arrive at the observed circular orbit unless it formed rather circular to begin with .