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We present simultaneous photometric and spectroscopic observations of sevenyoung and highly variable M dwarfs in star forming regions in Orion, conductedin 4 observing nights with FORS2 at ESO/VLT. All seven targets show significantphotometric variability in the I-band, with amplitudes between 0.1-0.8 mag, Thespectra, however, remain remarkably constant, with spectral type changes lessthan 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, inaddition to the continuum variations. Their behaviour is mostly consistent withthe presence of spots with temperature of $\sim300$ K above the photosphere andfilling factors between 0.2-0.4, in contrast to typical hot spots observed inmore massive stars. The remaining targets near $\epsilon$ Ori, likely to beolder, show eclipse-like lightcurves, no significant H$\alpha$ activity and arebetter represented by variable extinction due to circumstellar material.Interestingly, two of them show no evidence of infrared excess emission. Ourstudy shows that high-amplitude variability in young very low mass stars can becaused by different phenomena than in more massive T Tauri stars and canpersist when the disk has disappeared and accretion has ceased.

Variability in young very low mass stars: Two surprises from spectrophotometric monitoring