  Stellar winds are a crucial component of massive stars, but their exact
properties still remain uncertain. To shed some light on this subject, we have
analyzed an exceptional set of X-ray observations of zeya Pup, one of the
closest and brightest massive stars. The sensitive lightcurves that were
derived reveal two major results. On the one hand, a slow modulation of the
X-ray flux (with a relative amplitude of up to 15% over 16h in the 0.3--4.0keV
band) is detected. Its characteristic timescale cannot be determined with
precision, but amounts from one to several days. It could be related to
corotating interaction regions, known to exist in zeta Pup from UV
observations. Hour-long changes, linked to flares or to the pulsation activity,
are not observed in the last decade covered by the XMM observations; the 17h
tentative period, previously reported in a ROSAT analysis, is not confirmed
either and is thus transient, at best. On the other hand, short-term changes
are surprisingly small (<1% relative amplitude for the total energy band). In
fact, they are compatible solely with the presence of Poisson noise in the
data. This surprisingly low level of short-term variability, in view of the
embedded wind-shock origin, requires a very high fragmentation of the stellar
wind, for both absorbing and emitting features (>10^5 parcels, comparing with a
2D wind model). This is the first time that constraints have been placed on the
number of clumps in an O-type star wind and from X-ray observations.
