We observed spectroscopically the eclipsing binary system T-Cyg1-01385 in order to determine physical properties of the components .
The double-lined nature of the system is revealed for the first time and the radial velocities are obtained for both stars .
We have derived masses , radii and luminosities for both components .
Analyses of the radial velocities and the KeplerCam and the T r ES light curves yielded masses of M _ { 1 } =1.059 \pm 0.032 M _ { \odot } and M _ { 2 } =0.342 \pm 0.017 M _ { \odot } and radii of R _ { 1 } =1.989 \pm 0.022 R _ { \odot } and R _ { 2 } =0.457 \pm 0.013 R _ { \odot } .
Locations of the low-mass companion in the mass-radius and mass-effective temperature planes and comparison with the other low-mass stars show that the secondary star appears just at the transition from partially to fully convective interiors for the M dwarfs .
When compared to stellar evolution models , the luminosities and effective temperatures of the components are consistent with Z=0.004 and an age of about 6 Gyr .
A distance to the system was calculated as d=355 \pm 7 pc using the BV and JHK magnitudes .