A search for photometric rotation periods in low-mass stars and brown dwarfs in the Pleiades

We have photometrically monitored (Cousins I-c) eight low-mass stars and brown dwarfs that are probable members of the Pleiades. We derived rotation periods for two of the stars-HHJ 409 and CFHT-PL 8-to be 0.258 and 0.401 days, respectively. The masses of these stars ate near 0.4 and 0.08 M-., respectively; the latter is the second such object near the hydrogen-burning boundary for which a rotation period has been measured. We also observed HHJ 409 in V. The relative amplitude in the two bands shows that the spots in that star are about 200 K cooler than the stellar effective temperature of 3560 K and have a filling factor on the order of 13%. With one possible exception, the remaining stars in the sample do not show photometric variations larger than the mean error of measurement. We also examined the M9.5 V disk star 2MASS J0149090 + 295613, which had previously exhibited a strong flare event, but we did not detect any photometric variation.