2013MNRAS.431.3240N

Star formation theory predicts that short-period M-dwarf binaries with highly unequal-mass components are rare. First, the mass ratio of close binary systems is driven to unity due to the secondary preferentially accreting gas with high angular momentum. Secondly, both dynamical decay of multiple systems and interactions with tertiary stars that tighten the binary orbit will eject the lowest mass member. Generally, only the two most massive stars are paired after such interactions, and the frequency of tight unequal-mass binaries is expected to decrease steeply with primary mass. In this paper, we present the discovery of a highly unequal-mass eclipsing M-dwarf binary, providing a unique constraint on binary star formation theory and on evolutionary models for low-mass binary stars. The binary is discovered using high-precision infrared light curves from the United Kingdom Infrared Telescope (UKIRT) Wide Field Camera (WFCAM) Transit Survey and has an orbital period of 2.44d. We find stellar masses of M1 = 0.53(±0.02)M☉ and M2 = 0.143(±0.006)M☉ (mass ratio 0.27), and radii of R1 = 0.51(±0.01) R☉ and R2 = 0.174(±0.006) R☉. This puts the companion in a very sparsely sampled and important late M-dwarf mass regime. Since both stars will share the same age and metallicity and straddle the theoretical boundary between fully and partially convective stellar interiors, a comparison can be made to model predictions over a large range of M-dwarf masses using the same model isochrone. Both stars appear to have a slightly inflated radius compared to 1Gyr` model predictions for their masses, but future work is needed to properly account for the effects of star spots on the light-curve solution. A significant, subsynchronous, ∼2.56d signal with ∼2percent peak-to-peak amplitude is detected in the WFCAM light curve, which we attribute to rotational modulation of cool star spots. We propose that the subsynchronous rotation is either due to a stable star-spot complex at high latitude on the (magnetically active) primary (i.e. differential rotation), or additional magnetic braking, or interaction of the binary with a third body or circumbinary disc during its pre-main-sequence phase.