  Using Monte Carlo simulations and published radial velocity surveys we have
constrained the frequency and separation (a) distribution of very low mass star
(VLM) and brown dwarf (BD) binary systems. We find that simple Gaussian
extensions of the observed wide binary distribution, with a peak at 4 au and
0.6< sigma_{log(a/au)}<1.0, correctly reproduce the observed number of close
binary systems, implying a close (a<2.6 au) binary frequency of 17-30 per cent
and overall frequency of 32-45 per cent. N-body models of the dynamical decay
of unstable protostellar multiple systems are excluded with high confidence
because they do not produce enough close binary VLMs/BDs. The large number of
close binaries and high overall binary frequency are also completely
inconsistent with published smoothed particle hydrodynamical modelling and
argue against a dynamical origin for VLMs/BDs.
