We report on two photometric monitoring campaigns of Very Low Mass (VLM)
objects in the young open cluster around sigma Orionis. Our targets
were pre-selected with multi-filter photometry in a field of 0.36 square degree.
For 23 of these objects, spanning a mass range from 0.03 to 0.7 M_sun,
we detect periodic variability. Of these, 16 exhibit low-level variability,
with amplitudes of less than 0.2 mag in the I-band, which is mostly
well-approximated by a sine wave. These periodicities are probably caused by
photospheric spots co-rotating with the objects. In contrast, the remaining
variable targets show high-level variability with amplitudes ranging from
0.25 to 1.1 mag, consisting of a periodic light variation onto which short-term
fluctuations are superimposed. This variability pattern is very similar to the
photometric behaviour of solar-mass, classical T Tauri stars. Low-resolution
spectra of a few of these objects reveal strong Halpha and Ca-triplet emission,
indicative of ongoing accretion processes. This suggests that 5-7% of our targets
still possess a circumstellar disk. In combination with previous results for younger
objects, this translates into a disk lifetime of 3-4 Myr, significantly shorter
than for solar mass stars. The highly variable objects rotate on average slower
than the low-amplitude variables, which is expected in terms of a disk-locking
scenario. There is a trend towards faster rotation with decreasing mass, which
might be caused by shortening of the disk lifetimes or attenuation of magnetic fields.