ROSAT has allowed the detection in X–rays of a large fraction of M dwarfs both in young open clusters such as the Pleiades and α Persei, and in the older Hyades. No decline of the average X–ray luminosity occurs between α Per and the Pleiades, while a rather steep decay is seen between the latter and the Hyades. The similarity of the Pleiades and α Per M dwarfs X–ray activity distributions simply reflects the similarity in their rotation distributions. It is more difficult to understand, instead, why the Hyades are, on average, significantly underluminous with respect to the Pleiades, since, due to the long spin-down timescales for M dwarfs, a large fraction of moderate or even rapid rotators are still present in the Hyades.
Although fully convective stars as active as stars with a radiative core have been observed, based on the Hyades, there might be an indication for a slight drop of the average X–ray emission level below the fully convective boundary mass, indicating a possible loss of efficiency of the mechanism of magnetic field generation.
Stars with masses down to 0.13 M⊙ and 0.19 M⊙ have been detected in the Pleiades and the Hyades, respectively: These detections, together with that of a 0.04 M⊙ brown dwarf in the Chamaleon I star forming region and of very-low mass dwarfs in the field, support the idea that there is not a cut-off mass below which stars do not have coronae anymore.