  We present simultaneous photometric and spectroscopic observations of seven
young and highly variable M dwarfs in star forming regions in Orion, conducted
in 4 observing nights with FORS2 at ESO/VLT. All seven targets show significant
photometric variability in the I-band, with amplitudes between 0.1-0.8 mag, The
spectra, however, remain remarkably constant, with spectral type changes less
than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that
'fills in' absorption features. Three objects in the $\sigma$ Ori cluster (age
$\sim$3 Myr) exhibit strong H$\alpha$ emission and H$\alpha$ variability, in
addition to the continuum variations. Their behaviour is mostly consistent with
the presence of spots with temperature of $\sim300$ K above the photosphere and
filling factors between 0.2-0.4, in contrast to typical hot spots observed in
more massive stars. The remaining targets near $\epsilon$ Ori, likely to be
older, show eclipse-like lightcurves, no significant H$\alpha$ activity and are
better represented by variable extinction due to circumstellar material.
Interestingly, two of them show no evidence of infrared excess emission. Our
study shows that high-amplitude variability in young very low mass stars can be
caused by different phenomena than in more massive T Tauri stars and can
persist when the disk has disappeared and accretion has ceased.
