| Summary: | 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. © 2008 The Authors. Journal compilation © 2008 RAS. Accepted 2007 October 30. Received 2007 October 11; in original form 2007 April 17. Published Online: 16 January 2008. The authors would like to particularly thank the staff members at the Nordic Optical Telescope. They would also like to thank John Baldwin and Peter Warner for many helpful discussions. An anonymous referee gave suggestions which significantly strengthened the paper. NML acknowledges support from the UK Particle Physics and Astronomy Research Council (PPARC). This research has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. It also made use of NASA's Astrophysics Data System Bibliographic Services. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de ...
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