2013MNRAS.429...85C

The eclipsing binary T-Cyg1-12664 has been observed both spectroscopically and photometrically, and the radial velocities of both the components and the ground-based VRI light curves have been obtained. The Kepler R data and radial velocities for the system have been analysed simultaneously. Masses and radii have been obtained as 0.680±0.021M☉ and 0.613±0.007 R☉ for the primary star and 0.341±0.012M☉ and 0.897±0.012 R☉ for the secondary star. The distance to the system has been estimated as 127 ± 14 pc. The observed wave-like distortion at out-of-eclipse is modelled with two separate spots on the more massive star, which is also confirmed by the Ca ii K and H emission lines in its spectra. The locations of the components in the mass-radius and mass-effective temperature planes have been compared with the well-determined low-mass components of eclipsing binaries as well as with the theoretical models. While the radius of the primary star is consistent with the main-sequence stars, the radius of the less massive component appears to be 2.8 times larger than that of the main-sequence models. A comparison of the radii of low-mass stars with the models reveals that the observationally determined radii begin to deviate from the models with a mass of 0.27M☉, and suddenly reach to maximum deviation at a mass of 0.34M☉. Then, the deviations begin to decrease up to the solar mass. The maximum deviation, seen at a mass of about 0.34M☉, is very close to the mass of fully convective stars, as suggested by theoretical studies. A third star in the direction of the eclipsing pair has been detected from our VRI images. The observed infrared excess of the binary most probably arises from this star, which can be radiated mostly in the infrared bands.