The eclipsing binary T-Cyg1-12664 was observed both spectroscopically and photometrically. Radial velocities of both components and ground-based VRI light curves were obtained. The Keplers R-data and radial velocities for the system were analysed simultaneously. Masses and radii were obtained as 0.680$pm$0.021 M$_{odot}$ and 0.613$pm$0.007 R$_{odot}$for the primary and 0.341$pm$0.012M$_{odot}$ and 0.897$pm$0.012R$_{odot}$ for the secondary star. The distance to the system was estimated as 127$pm$14 pc. The observed wave-like distortion at out-of-eclipse is modeled with two separate spots on the more massive star, which is also confirmed by the Ca {sc ii} K and H emission lines in its spectra. Locations of the components in the mass-radius and mass-effective temperature planes were compared with the well-determined eclipsing binaries low-mass components as well as with the theoretical models. While the primary stars radius 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. 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 0.27 Msun and suddenly reaches to maximum deviation at a mass of 0.34 Msun. Then, the deviations begin to decrease up to the solar mass. The maximum deviation seen at a mass of about 0.34 Msun 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 is most probably arisen from this star which may be radiated mostly in the infrared bands.
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