Context : The observational mass-radius relation of main sequence stars with masses between \sim 0.3 and 1.0 M _ { \sun } reveals deviations between the stellar radii predicted by models and the observed radii of stars in detached binaries .

Aims : We generate an accurate physical model of the low-mass eclipsing binary T-Cyg1-12664 in the Kepler mission field to measure the physical parameters of its components and to compare them with the prediction of theoretical stellar evolution models .

Methods : We analyze the Kepler mission light curve of T-Cyg1-12664 to accurately measure the times and phases of the primary and secondary eclipse .
In addition , we measure the rotational period of the primary component by analyzing the out-of-eclipse oscillations that are due to spots .
We accurately constrain the effective temperature of the system using ground-based absolute photometry in B , V , R _ { C } , and I _ { C } .
We also obtain and analyze VR _ { C } I _ { C } differential light curves to measure the eccentricity and the orbital inclination of the system , and a precise T _ { eff } ratio .
From the joint analysis of new radial velocities and those in the literature we measure the individual masses of the stars .
Finally , we use the PHOEBE code to generate a physical model of the system .

Results : T-Cyg1-12664 is a low eccentricity system , located d =360 \pm 22 pc away from us , with an orbital period of P = 4.1287955 ( 4 ) days , and an orbital inclination i =86.969 \pm 0.056 degrees .
It is composed of two very different stars with an active G6 primary with T _ { eff 1 } =5560 \pm 160 K , M _ { 1 } =0.680 \pm 0.045 M _ { \sun } , R _ { 1 } =0.799 \pm 0.017 R _ { \sun } , and a M3V secondary star with T _ { eff 2 } =3460 \pm 210 K , M _ { 2 } =0.376 \pm 0.017 M _ { \sun } , and R _ { 2 } =0.3475 \pm 0.0081 R _ { \sun } .

Conclusions : The primary star is an oversized and spotted active star , hotter than the stars in its mass range .
The secondary is a cool star near the mass boundary for fully convective stars ( M \sim 0.35 M _ { \sun } ) , whose parameters appear to be in agreement with low-mass stellar model .