Strömgren four-color photometry of X-ray active late-type stars: Evidence for activity-induced deficiency in the indexF. Morale, G. Micela, F. Favata and S. SciortinoAstron. Astrophys. Suppl. Ser., 119 3 (1996) 403-412DOI: https://doi.org/10.1051/aas:1996255