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We present high-precision photometry on 107 variable low-mass stars and browndwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-bandphotometric monitoring within two fields, encompassing 153 confirmed orcandidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot.We are sensitive to brightness changes on time scales from 10 minutes to twoweeks with amplitudes as low as 0.004 magnitudes, and find variability on thesetime scales in nearly 70% of cluster members. We identify both periodic andaperiodic modes of variability, as well as semi-periodic rapid fading eventsthat are not accounted for by the standard explanations of rotationalmodulation of surface features or accretion. We have incorporated both opticaland infrared color data to uncover trends in variability with mass andcircumstellar disks. While the data confirm that the lowest-mass objects(M<0.2M_\odot) rotate more rapidly than the 0.2-0.5 M_\odot members, they donot support a direct connection between rotation rate and the presence of adisk. Finally, we speculate on the origin of irregular variability in clustermembers with no evidence for disks or accretion.

Precision photometric monitoring of very low mass Sigma Orionis cluster members: variability and rotation at a few Myr