Supersaturation in X-ray emission of clusters stars

A population of cool dwarfs with extreme rotational velocities (v sini > 100 km/sec) is present in young open clusters. ROSAT observations have shown that these very fast rotators exhibit a level of X-ray activity a factor of 3-5 below the saturated level which is usually observed (both in X-rays and other magnetic activity indicators) for `normally' fast rotators. This phenomenon has been denominated "Supersaturation". W UMa contact binaries seem to be supersaturated as well, while the scatter in the rotation-activity relation for RS CVn and BY Dra binaries does not allow us to clearly discern whether they exhibit supersaturation or not. Supersaturation is not seen in Halpha for Alpha Per supersaturated stars. Two alternative lines of interpretation are discussed.