  We present radial velocities for 38 low-mass candidate members of the sigma
Orionis young group. We have measured their radial velocities by
cross-correlation of high resolution (R~6000) AF2/WYFFOS spectra of the gravity
sensitive NaI doublet at 8183, 8195Angstroms. The total sample contained 117
objects of which 54 have sufficient signal-to-noise to detect NaI at an
equivalent width of 3Angstroms, however we only detect NaI in 38 of these. This
implies that very low-mass members of this young group display weaker NaI
absorption than similarly aged objects in the Upper Scorpius OB association. We
develop a technique to assess membership using radial velocities with a range
of uncertainties that does not bias the selection when large uncertainties are
present. The resulting membership probabilities are used to assess the issue of
exclusion in photometric selections, and we find that very few members are
likely to be excluded by such techniques.
  We also assess the level of contamination in the expected pre-main sequence
region of colour-magnitude space brighter than I = 17. We find that
contamination by non-members in the expected PMS region of the colour-magnitude
diagram is small. We conclude that although radial velocity alone is
insufficient to confirm membership, high signal-to-noise observations of the
NaI doublet provide the opportunity to use the strength of NaI absorption in
concert with radial velocities to asses membership down to the lowest masses,
where Lithium absorption no longer distinguishes youth.
