We present new , contemporaneous HST STIS and XMM-Newton 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 Per .
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 ~ { } \lambda 1718 and Si iv ~ { } \lambda 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-Newton 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 ~ { } \lambda 1402 , N iv ~ { } \lambda 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 ^ { \circ } , on the surface of the star .
We note that the presence and persistent of two spots separated by 180 ^ { \circ } 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 .