2006MNRAS.367..407R

We present simultaneous UV, G, R and I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf with an amplitude in the UV of at least 6 mag. This is one of the strongest detections ever of an optical flare in an M star and one of the first in an ultracool dwarf (spectral types later than about M7). Four intermediate-strength flares (Δm_UV< 4mag) were found in this and three other targets. For the whole sample we deduce a flare probability of 0.013 (rate of 0.022h^–1), and 0.049 (0.090h^–1) for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong Hα line emission. We also confirm our previously found rotation period for 2M1707+64 and determine it more precisely to be 3.619±0.015h.