Four ultra-short-period eclipsing M-dwarf binaries in the WFCAM Transit Survey

We report on the discovery of four ultra-short-period (P = 0.18?d) eclipsing M-dwarf binaries in the Wide-Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main-sequence binary system, and are all significantly below the sharp period cut-off at P similar to 0.22 ?d as seen in binaries of earlier-type stars. The shortest-period binary consists of two M4-type stars in a P = 0.112 ?d orbit. The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260?000 stellar light curves, including over 10?000 M-dwarfs down to J = 18?mag, yielding 25 binaries with P = 0.23?d. In a popular paradigm, the evolution of short-period binaries of cool main-sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P similar to 0.22?d period cut-off is explained as being due to time-scales that are too long for lower-mass binaries to decay into tighter orbits. Our discovery of low-mass binaries with significantly shorter orbits implies that either these time-scales have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M-dwarf binaries is different from that of earlier-type main-sequence stars.