Star formation theory predicts that short-period M-dwarf binaries with highly unequal-mass components are rare .
Firstly , 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 WFCAM Transit Survey ( WTS ) and has an orbital period of 2.44 d. We find stellar masses of M _ { 1 } = 0.53 \pm 0.02 M _ { \odot } and M _ { 2 } = 0.143 \pm 0.006 M _ { \odot } ( mass ratio 0.27 ) , and radii of R _ { 1 } = 0.51 \pm 0.01 R _ { \odot } and R _ { 2 } = 0.174 \pm 0.006 R _ { \odot } .
This puts the companion in a very sparsely sampled and important late M-dwarf mass-regime .
Since both stars 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 1 Gyr 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 , \sim 2.56 d signal with \sim 2 % 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 to additional magnetic braking , or to interaction of the binary with a third body or circumbinary disk during its pre-main-sequence phase .