  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 M1 = 0.53 (0.02)
Msun and M2 = 0.143 (0.006) Msun (mass ratio 0.27), and radii of R1 = 0.51
(0.01) Rsun and R2 = 0.174 (0.006) Rsun. 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, ~2.56 d signal with ~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.
