Abstract
Mg IIh and k fluxes are given for late-type dwarfs whose rotation periods are known. A good correlation between an activity parameter Rhk (represented by the ratio of Mg II h and k luminosity to bolometric luminosity) versus the Rossby-number is found for main-sequence stars F to late M. Both single and binary stars are found to follow the same relation. A saturation of Rhk occurs at rotation periods of approximately 4 day, it is suggested that stars whose rotation period is close to or shorter than this value are completely covered by active plage regions, although an alternative explanation may be saturation of the dynamo itself.

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