  Searches for companions of brown dwarfs by direct imaging mainly probe
orbital separations > 3-10 AU. On the other hand, previous radial velocity
surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6
AU. It has been speculated that the peak of the separation distribution of
brown dwarf binaries lies right in the unprobed range. This work extends
high-precision radial velocity surveys of brown dwarfs for the first time out
to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary
frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in ChaI was
determined: 18% for the whole sample and 10% for the subsample of ten brown
dwarfs and VLMS (M < 0.1 Msun). Two spectroscopic binaries were confirmed, the
brown dwarf candidate ChaHa8 (previously discovered by Joergens & Mueller) and
the low-mass star CHXR74. Since their orbital separations appear to be 1 AU or
greater, the binary frequency at < 1 AU might be less than 10%. Now for the
first time companion searches of (young) brown dwarfs cover the whole orbital
separation range, and the following observational constraints for models of
brown dwarf formation can be derived: (i) the frequency of brown dwarf and very
low-mass stellar binaries at < 3 AU does not significantly exceed that at > 3
AU; i.e. direct imaging surveys do not miss a significant fraction of brown
dwarf binaries; (ii) the overall binary frequency of brown dwarfs and very
low-mass stars is 10-30 %; (iii) the decline in the separation distribution of
brown dwarfs towards smaller separations seems to occur between 1 and 3 AU;
(iv) the observed continuous decrease in the binary frequency from the stellar
to the substellar regime is confirmed at < 3 AU providing further evidence of a
continuous formation mechanism from low-mass stars to brown dwarfs.
