X-ray luminosity function of the Pleiades, Hyades, Her-Lyr (asterisks), and Ursa Major (squares). The logarithm of the luminosity measured by ROSAT is displayed with respect to the Kaplan-Meier estimator (Meier 1958; Feigelson & Nelson 1985) as an empirical cumulative distribution function of the X-ray luminosity. The X-ray data of the Pleiades and the Hyades was taken from the WEBDA database (Mermilliod 1996). For the Ursa Major and Her-Lyr stars, data from the ROSAT All-Sky Bright Source Catalog (Voges et al. 1999) and the Second ROSAT PSPC Catalog (Rosat Consortium 2000) was used. Ursa Major members were taken from the rather comprehensive study of King et al. (2003).