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ABSTRACT We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light-curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type-colour relations, and over 9600 dwarfs ( J < 17) with spectral types later than K7 were found. The light curves of the late-type sample are searched for periodicity using a Lomb-Scargle periodogram analysis. A total of 68 periodic variable M dwarfs are found in the sample with periods ranging from 0.16 to 90.33 d, with amplitudes in the range of ∼0.009 to ∼0.115 in the J band. We simulate active M dwarfs with a range of latitude-independent spot coverages and estimate a periodically variable fraction of 1-3 per cent for stars where spots cover more than 10 per cent of the star's surface. Our simulated spot distributions indicate that operating in the J band, where spot contrast ratios are minimized, enables variability in only the most active of stars to be detected. These findings affirm the benefits of using the J band for planetary transit searches compared to visible bands. We also serendipitously find a Δ J > 0.2 mag flaring event from an M4V star in our sample. [ABSTRACT FROM AUTHOR]

ABSTRACT Using data from the Hungarian-made Automated Telescope Network (HATNet) survey for transiting exoplanets, we measure photometric rotation periods for 368 Pleiades stars with 0.4 ≲ M≲ 1.3 M⊙. We detect periodic variability for 74 per cent of the cluster members in this mass range that are within our field-of-view, and 93 per cent of the members with 0.7 ≲ M≲ 1.0 M⊙. This increases, by a factor of 5, the number of Pleiades members with measured periods. We compare these data to the rich sample of spectroscopically determined projected equatorial rotation velocities ( v sin i) available in the literature for this cluster. Included in our sample are 14 newly identified probable cluster members which have proper motions, photometry and rotation periods consistent with membership. For stars with M≳ 0.85 M⊙ the rotation periods, v sin i and radius estimates are consistent with the stars having an isotropic distribution of rotation axes, if a moderate differential rotation law is assumed. For stars with M≲ 0.85 M⊙, the inferred sin i values are systematically larger than 1.0. These observations imply that the combination of measured parameters P( v sin i)/ R is too large by ∼ 24 per cent for low-mass stars in this cluster. By comparing our new mass-period relation for the Pleiades to the slightly older cluster M35, we confirm previous indications that the spin-down stalls at ∼ 100 Myr for the slowest rotating stars with 0.7 ≲ M≲ 1.1 M⊙- a fact which may indicate that the internal transport of angular momentum is inefficient in slowly rotating solar-mass stars. [ABSTRACT FROM AUTHOR]