CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi   Persei?
  We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star xi Per, together with contemporaneous H alpha observations. The X-ray spectrum of this star is similar to other single O stars, and not pathological in any way. Its UV wind lines are known to display cyclical time variability, with a period of 2.086 days, which is thought to be associated with co-rotating interaction regions (CIRs). We examine the Chandra and H alpha data for variability on this time scale. We find that the X-rays vary by about 15% over the course of the observations and that this variability is out of phase with variable absorption on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV absorption associated with CIRs). While not conclusive, both sets of data are consistent with models where the CIRs are either a source of X-rays or modulate them. 
D. Massa, L. Oskinova, A. W. Fullerton, R. K. Prinja, D. A. Bohlender,   N. D. Morrison, M. Blake and W. Pych
astro-ph.SR