We report on the results of a time-series photometric survey of M50 (NGC 2323), a ~130 Myr open cluster, carried out using the CTIO 4m Blanco telescope and Mosaic-II detector as part of the Monitor project. Rotation periods were derived for 812 candidate cluster members over the mass range 0.2 <~ M/Msol <~ 1.1. The rotation period distributions show a clear mass-dependent morphology, statistically indistinguishable from those in NGC 2516 and M35 taken from the literature. Due to the availability of data from three observing runs separated by ~10 and 1 month timescales, we are able to demonstrate clear evidence for evolution of the photometric amplitudes, and hence spot patterns, over the 10 month gap, although we are not able to constrain the timescales for these effects in detail due to limitations imposed by the large gaps in our sampling, preventing use of the phase information.
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