Stromgren four-color photometry of X-ray active late-type stars: Evidence for activity-induced deficiency in the m(1) index

We present the results of a uvby-beta photometric study of a sample of active late-type stars (F-K) selected from the Einstein Extended Medium Sensitivity Survey. Our work shows the presence in the sample of a star population with the photometric index c(1) typical of main sequence stars and an unexpected deficiency in m(1) index. Stars with more anomalous values of m(1) have also very high values of f(x)/f(v) and X-ray surface fluxes, near the ''saturation'' limit observed in the most active stars and similar to the flux observed in solar active regions. We discuss these results in the light of similar results found in the Sun comparing mi indices in quiet and active regions and in other samples of active stars.