  VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with
previously known data, have been used to establish the membership and the
properties of a sample of low-mass candidate members of the sigma Orionis
cluster with masses spanning from 1 Msun down to about 0.013 Msun (i.e.,
deuterium-burning mass limit). We have observed K-band infrared excess and
remarkably intense H(alpha) emission in various cluster members, which, in
addition to the previously detected forbidden emision lines and the presence of
LiI in absorption at 6708 A, have allowed us to tentatively classify sigma
Orionis members as classical or weak-line TTauri stars and substellar analogs.
Variability of the H(alpha) line has been investigated and detected in some
objects. Based on the K-band infrared excesses and the intensity of H(alpha)
emission, we estimate that the minimum disk frequency of the sigma Orionis
low-mass population is in the range 5-12%.
