  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 $M_1 =
1.0886\pm0.0031\, M{\odot}$, $M_2 = 0.8292 \pm 0.0026\, M{\odot}$, $R_1 =
1.4203\pm 0.0058\, R{\odot}$, $R_2 = 0.7997 \pm 0.0015\, R{\odot}$, $L_1 =
1.85\pm 0.15\, L{\odot}$, $L_2 = 0.292 \pm 0.018\, L{\odot}$ 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 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 TWIN is insufficient
to arrive at the observed circular orbit unless it formed rather circular to
begin with.
