2007ApJ...663..573B

We derive masses and radii for both components in the single-lined eclipsing binary HAT-TR-205-013, which consists of an F7 V primary and a late M dwarf secondary. The system's period is short, P=2.230736±0.000010 days, with an orbit indistinguishable from circular, e=0.012±0.021. We demonstrate generally that the surface gravity of the secondary star in a single-lined binary undergoing total eclipses can be derived from characteristics of the light curve and spectroscopic orbit. This constrains the secondary to a unique line in the mass-radius diagram, with M/R² = constant. For HAT-TR-205-013, we assume the orbit has been tidally circularized and that the primary's rotation has been synchronized and aligned with the orbital axis. Our observed line broadening, V_rotsini_rot=28.9±1.0 km/s, gives a primary radius of R_A=1.28±0.04 R_☉. Our light-curve analysis leads to the radius of the secondary, R_B=0.167±0.006 R_☉, and the semimajor axis of the orbit, a=7.54±0.30 R_☉=0.0351±0.0014 AU. Our single-lined spectroscopic orbit and the semimajor axis then yield the individual masses M_B=0.124±0.010 M_☉ and M_A=1.04±0.13 M_☉. Our result for HAT-TR-205-013 B lies above the theoretical mass-radius models from the Lyon group, consistent with results from double-lined eclipsing binaries. The method we describe offers the opportunity to study the very low end of the stellar mass-radius relation.