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We explore the rotation and activity of very low mass (VLM) objects by meansof a photometric variability study. Our targets in the vicinity of Epsilon Oribelong to the OriOB1b population in the Orion star-forming complex. In thisregion we selected 143 VLM stars and brown dwarfs (BDs), whose photometry inRIJHK is consistent with membership of the young population. The variability ofthese objects was investigated using a densely sampled I-band time seriescovering four consecutive nights with altogether 129 data points per object.Our targets show three types of variability: Thirty objects, including nineBDs, show significant photometric periods, ranging from 4h up to 100h, which weinterpret as the rotation periods. Five objects, including two BDs, exhibitvariability with high amplitudes up to 1 mag which is at least partlyirregular. This behaviour is most likely caused by ongoing accretion andconfirms that VLM objects undergo a T Tauri phase similar to solar-mass stars.Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. Therotation periods show dependence on mass, i.e. the average period decreaseswith decreasing object mass, consistent with previously found mass-periodrelationships in younger and older clusters. The period distribution of BDsextends down to the breakup period, where centrifugal and gravitational forcesare balanced. Combining our BD periods with literature data, we found that thelower period limit for substellar objects lies between 2h and 4h, more or lessindependent of age. Contrary to stars, these fast rotating BDs seem to evolveat constant rotation period from ages of 3 Myr to 1 Gyr, in spite of thecontraction process. Thus, they should experience strong rotational braking.Comment: 17 pages, 13 figures, A&A, in pres

Rotation and variability of very low mass stars and brown dwarfs near ϵ Ori