  We report on the analysis of 2MASS near-infrared data of a sample of low-mass
stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster
membership have been spectroscopically confirmed using the Li I spectral line.
We find little evidence in the JHKs colour-colour diagram for near-infrared
excess emission for these cluster members. By comparison with model
expectations, at most 2 out of 34 stars show (H-K) colour consistent with a
near-infrared excess. This scarcity of near-infrared signatures of
circumstellar disks in the lower-mass and substellar regimes of this cluster
contrasts with findings in younger clusters, hinting at an age dependence of
the disk frequency. Taking into account the apparent cluster age, our result
supports the idea of a relatively fast (few Myr) disk dissipation and extends
this conclusion to the substellar regime. We also find some evidence that, in
this cluster, the disk frequency as measured by the Ks-band excess may be mass
dependent.
