Hα equivalent width vs. J-band magnitude diagram in σ Orionis. Radii of large (black) circles are proportional to the peak-to-peak amplitude of known photometric variability in the optical. Filled (red) objects are objects investigated here. Dotted horizontal lines denote EW(Hα) = 0, –10, and –100 Å, from bottom to top. The cluster stellar-substellar boundary lies at about J = 14.5 mag.