The LuckyCam survey for very low mass binaries - II. 13 new M4.5-M6.0 binaries

We present results from a high angular resolution survey of 77 very low mass (VLM) binary systems with 6.0 <= V - K colour <= 7.5 and proper motion >= 0.15 arcsec yr(-1). 21 VLM binaries were detected, 13 of them new discoveries. The new binary systems range in separation between 0.18 and 1.3 arcsec. The distance-corrected binary fraction is 13.6(-4)(+6.5) per cent, in agreement with previous results. Nine of the new binary systems have projected separations > 10 au, including a new wide VLM binary with 27 au projected orbital separation. One of the new systems forms two components of a 2300 au separation triple system. We find that the projected separation distribution of the binaries with V - K < 6.5 in this survey appears to be different from that of redder (lower mass) objects, suggesting a possible rapid change in the orbital radius distribution at around the M5 spectral type. The target sample was also selected to investigate X-ray activity among VLM binaries. There is no detectable correlation between excess X-ray emission and the frequency and binary properties of the VLM systems.