  Searches for companions of brown dwarfs by direct imaging probe mainly
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 if the peak of the separation distribution of brown
dwarf binaries lies right in the unprobed range. Very recent work for the first
time extends high-precision radial velocity surveys of brown dwarfs out to 3 AU
(Joergens 2008, A&A). 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: it is 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, these are the brown dwarf candidate ChaHa8, 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 is not significantly exceeding 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 of the separation distribution of brown
dwarfs towards smaller separations seem to occur between 1 and 3 AU; (iv) the
observed continuous decrease of the binary frequency from the stellar to the
substellar regime is confirmed at < 3 AU providing further evidence for a
continuous formation mechanism from low-mass stars to brown dwarfs.

Astrophysics