The issue of the variability of the coronal emission is a current problem of
stellar physics. Indeed while the Sun exhibits large variability in the
X-ray energy band, with amplitude variations of about one order
of magnitude during its rotational modulation an its 11-years cycle,
stars observed more than once with ROSAT and EINSTEIN vary (excluding
flares) at most by a factor two.
Here, we present a preliminary report of a comprehensive variability analysis of X-ray observations of Pleiades clusters. Taking advantage of IPC obervations (obtained in 1981), of pointed PSPC observations (obtained in 1991) and the more recent HRI/ROSAT observations (1994/1996) we have explored a range of time scales up to typical time scale comparable with that of the solar cycle.