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 br own dwarfs that are probable members of the Pleiades. We derived rotation p eriods for two of the stars-HHJ 409 and CFHT-PL 8-to be 0.258 and 0.401 day s, respectively. The masses of these stars ate near 0.4 and 0.08 M-., respe ctively; the latter is the second such object near the hydrogen-burning bou ndary for which a rotation period has been measured. We also observed HHJ 4 09 in V. The relative amplitude in the two bands shows that the spots in th at star are about 200 K cooler than the stellar effective temperature of 35 60 K and have a filling factor on the order of 13%. With one possible excep tion, 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 flar e event, but we did not detect any photometric variation.