  We present new, contemporaneous HST STIS and XMM observations of the O7
III(n)((f)) star xi Per. We supplement the new data with archival IUE spectra,
to analyze the variability of the wind lines and X-ray flux of xi Pper. The
variable wind of this star is known to have a 2.086 day periodicity. We use a
simple, heuristic spot model which fits the low velocity (near surface) IUE
wind line variability very well, to demonstrate that the low velocity
absorption in the new STIS spectra of N IV 1718 and Si IV 1402 vary with the
same 2.086 day period. It is remarkable that the period and amplitude of the
STIS data agree with those of the IUE spectra obtained 22 years earlier. We
also show that the time variability of the new XMM fluxes are also consistent
with the 2.086 day period. Thus, our new, multi-wavelength coordinated
observations demonstrate that the mechanism which causes the UV wind line
variability is also responsible for a significant fraction of the X-rays in
single O stars. The sequence of events for the multi-wavelength light curve
minima is: Si IV 1402, N IV 1718, and X-ray flux, each separated by a phase of
about 0.06 relative to the 2.086 day period. Analysis of the X-ray fluxes shows
that they become softer as they weaken. This is contrary to expectations if the
variability is caused by periodic excess absorption. Further, the high
resolution X-ray spectra suggest that the individual emission lines at maximum
are more strongly blue shifted.
  If we interpret the low velocity wind line light curves in terms of our
model, it implies that there are two bright regions, i.e., regions with less
absorption, separated by 180 deg, on the surface of the star. We note that the
presence and persistent of two spots separated by 180 deg suggests that a weak
dipole magnetic field is responsible for the variability of the UV wind line
absorption and X-ray flux in xi Per.
