ROSAT and AB Doradus: the first five years

Five and a half years of data from an extensive ROSAT X-ray monitoring program devoted to the young Kstar AB Dor are presented. Begun in mid-1990, this program is aimed at the study of the long-term behaviour of coronal flux levels in this very active star. We compare the X-ray data with 17 years of V-band brightness monitoring that shows a 10-year decline between 1978 and 1989 and a subsequent rise phase. In contrast, the X-ray flux, which is very variable on time scales of minutes to weeks, exhibits no pronounced long-term trend over the 5 1/2 years of the program. This supports the concept of a saturated corona. Making use of the ROSAT all-sky survey data and data from the most extensive among the available pointed observationswe also discuss the short-term variablity in relation to the rotational time scale. We find evidence for a partial rotational modulation of the X-ray flux implying structural inhomogeneities in this saturated corona.