  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 Kepler's 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 star's 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.
