Discovery and Precise Characterization by the MEarth Project of LP 661-13, an Eclipsing Binary Consisting of Two Fully Convective Low-mass Stars.

We report the detection of stellar eclipses in the LP 661-13 system. We present the discovery and characterization of this system, including high-resolution spectroscopic radial velocities and a photometric solution spanning two observing seasons. LP 661-13 is a low-mass binary system with an orbital period of days at a distance of 24.9 ± 1.3 parsecs. LP 661-13A is a 0.30795 ± 0.00084 M⊙ star, while LP 661-13B is a 0.19400 ± 0.00034 M⊙ star. The radius of each component is 0.3226 ± 0.0033 R⊙ and 0.2174 ± 0.0023 R⊙, respectively. We detect out-of-eclipse modulations at a period slightly shorter than the orbital period, implying that at least one of the components is not rotating synchronously. We find that each component is slightly inflated compared to stellar models, and that this cannot be reconciled through age or metallicity effects. As a nearby eclipsing binary system, where both components are near or below the full-convection limit, LP 661-13 will be a valuable test of models for the structure of cool dwarf stars. [ABSTRACT FROM AUTHOR]