A turn-offdetached binary star V568 Lyr in the Kepler field of the oldest open cluster (NGC 6791) in the Galaxy

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 M1 = 1.0886 ± 0.0031 M?, M2 = 0.8292 ± 0.0026 M?, R1 = 1.4203 ± 0.0058 R?, R2 = 0.7997 ± 0.0015 R?, L1 = 1.85 ± 0.15 L?, L2 = 0.292 ± 0.018 L? and their separation is a = 31.060 ± 0.025 R?. The distance to NGC 6791 is determined to be 4.260 ± 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. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.