We present the results of a combination of new stellar rotation periods and extensiveinformation about membership in the young open clusters M 35 and M 34. The observations showthat late-type members of both M 35 and M 34 divide into two distinct groups, each with a differentdependence of rotation on mass (color). We discuss these new results in the context of existingrotation data for cool stars in older clusters, with a focus on the dependence of rotation on massand age. We mention briefly tests of rotation as an "astronomical clock" (gyrochronology), and ourplans to use the Kepler space mission to push observations of stellar rotation periods beyond the ageof the Hyades and the Sun.
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