  We have studied the photometric variability of very young brown dwarfs and
very low-mass stars (masses well below 0.2 M_sun) in the ChaI star forming
region. We have determined photometric periods in the Gunn i and R band for the
three M6.5-M7 type brown dwarf candidates ChaHa2, ChaHa3 and ChaHa6 of 2.2 to
3.4 days. These are the longest photometric periods found for any brown dwarf
so far. If interpreted as rotationally induced they correspond to moderately
fast rotational velocities, which is fully consistent with their v sini values
and their relatively large radii. We have also determined periods for the two
M5-M5.5 type very low-mass stars B34 and CHXR78C. In addition to the Gunn i and
R band data, we have analysed JHK_s monitoring data of the targets, which have
been taken a few weeks earlier and confirm the periods found in the optical
data. Upper limits for the errors in the period determination are between 2 and
9 hours. The observed periodic variations of the brown dwarf candidates as well
as of the T Tauri stars are interpreted as modulation of the flux at the
rotation period by magnetically driven surface features, on the basis of a
consistency with v sini values as well as (R-i) color variations typical for
spots. Furthermore, the temperatures even for the brown dwarfs in the sample
are relatively high (>2800K) because the objects are very young. Therefore, the
atmospheric gas should be sufficiently ionized for the formation of spots on
one hand and the temperatures are too high for significant dust condensation
and hence variabilities due to clouds on the other hand.
