Membership, binarity and accretion among very low-mass stars and brown dwarfs of the Sigma Orionis cluster

Intermediate resolution (R=7000) spectroscopy is presented for 76 photometrically selected very low mass (0.04<M<0.3M_{sun}) candidate members of the Sigma Orionis cluster. More than two thirds appear to be genuine cluster members on the basis of Li I absorption, weak Na I features and radial velocities. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only 6 objects appear to be certain non-members, however a substantial subset of 13 have ambiguous or contradictory indications of membership and lack Li absorption. Four candidate binary cluster members are identified. Consideration of sampling and precision leads us to conclude that either the fraction of very low mass stars and brown dwarfs in small separation (a