We have obtained multi-epoch , high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs in the young clusters around \sigma Ori and \lambda Ori .
We find that 196 targets are cluster members based on their radial velocity , the equivalent width of their Na I 8200 lines and the spectral type from their TiO band strength .
We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its line width and shape .
Of these , 6 are double-lined spectroscopic binaries ( SB2 ) where the components of the binary are of comparable brightness .
The others are single-lined binaries ( SB1 ) in which the companion is faint or the spectra of the stars are blended .
There are 3 narrow-lined SB1 binaries in our sample for which the companion is more than 2.5 magnitudes fainter than the primary .
This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1 .
The sample covers the magnitude range \mbox { $ { I } _ { C } $ } = 14 – 18.9 ( mass \approx 0.55 \ - - 0.03 \mbox { $ { M } _ { \sun } $ } ) , but all of the binary stars are brighter than \mbox { $ { I } _ { C } $ } = 16.6 ( mass \approx 0.12 \mbox { $ { M } _ { \sun } $ } ) and 10 are brighter than \mbox { $ { I } _ { C } $ } = 15.5 ( mass \approx 0.23 \mbox { $ { M } _ { \sun } $ } ) .
There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude .
We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of { I } _ { C } magnitude with more than 99 percent confidence .
The spectroscopic binary fraction for stars more massive than about 0.1 \mbox { $ { M } _ { \sun } $ } ( \mbox { $ { I } _ { C } $ } < 16.9 ) is f _ { bright } = 0.095 ^ { +0.012 } _ { -0.028 } .
The 90 percent confidence upper limit to the spectroscopic binary fraction for very low mass ( VLM ) stars ( mass < 0.1 \mbox { $ { M } _ { \sun } $ } ) and brown dwarfs ( BDs ) is f _ { faint } < 7.5 percent .
The hypothesis that f _ { bright } and f _ { faint } are equal can be rejected with 90 percent confidence .
The average detection probability for our survey is 50 percent or more for binaries with separations up to 0.28 au for stars with \mbox { $ { I } _ { C } $ } < 16.9 and 0.033 au for the fainter stars in our sample .
We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and brown dwarfs when compared to more massive stars in the same star-forming region .
This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semi-major axes , or both .