We report on the discovery of four ultra-short period ( P \leq 0.18 days ) eclipsing M-dwarf binaries in the WFCAM Transit Survey .
Their orbital periods are significantly shorter than of any other known main-sequence binary system , and are all significantly below the sharp period cut-off at P \sim 0.22 days as seen in binaries of earlier type stars .
The shortest-period binary consists of two M4 type stars in a P = 0.112 day orbit .
The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260,000 stellar lightcurves , including over 10,000 M-dwarfs down to J=18 mag , yielding 25 binaries with P \leq 0.23 days .
In a popular paradigm , the evolution of short period binaries of cool main-sequence stars is driven by loss of angular momentum through magnetised winds .
In this scheme , the observed P \sim 0.22 day period cut-off is explained as being due to timescales 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 timescales have been overestimated for M-dwarfs , e.g .
due to a higher effective magnetic activity , or that the mechanism for forming these tight M-dwarf binaries is different from that of earlier type main-sequence stars .