We present simultaneous , multiband photometric monitoring of 19 field dwarfs covering most of the M spectral sequence ( M2–M9 ) .
Significant variability was found in seven objects in at least one out of the three channels I , R and G. Periodic variability was tested with a CLEAN power spectral analysis .
Two objects , LHS370 ( M5V ) and 2M1707+64 ( M9V ) , show periods of 5.9 \pm 2.0 and 3.65 \pm 0.1 hours respectively .
On account of the agreement with the typical values of v \sin i published for M dwarfs ( ) , we claim these to be the objects ’ rotation periods .
Three further objects show possible periods of a few hours .
Comparing the variability amplitude in each channel with predictions based on the synthetic spectra of , we investigated the source of variability in LHS370 and 2M1707+64 .
For the latter , we find evidence for the presence of magnetically-induced cool spots at a temperature contrast of 4 - 8 \% , with a projected surface coverage factor of less than 0.075 .
Moreover , we can rule out dust clouds ( as represented by the COND or DUSTY models ) as the cause of the variability .
No conclusion can be drawn in the case of LHS370 .
Comparing the frequency of occurrence of variability in this and various L dwarf samples published over the past few years , we find that variability is more common in field L dwarfs than in field M dwarfs ( for amplitudes larger than 0.005 { mag } on timescales of 0.5 to 20 hours ) .
Using the homogeneous data sets of this work and , we find fractions of variable objects of 0.21 \pm 0.11 among field M dwarfs and 0.70 \pm 0.26 among field L dwarfs ( and 0.29 \pm 0.13 , 0.48 \pm 0.12 respectively if we take into account a larger yet more inhomogeneous sample ) .
This is marginally significant ( 2 \sigma deviation ) and implies a change in the physical nature and/or extent of surface features when moving from M to L dwarfs .