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The Milky Way and the Magellanic Clouds contain a dozen or so X-ray binary systems in which the primary is a massive hot star with a stellar wind and the secondary is a neutron star or black hole. The X-ray emission results from capture of gas by the compact object, in some cases from the stellar wind of the primary and in some cases through Roche lobe overflow. In either case, the X-ray source will modify the hydrodynamics of the wind by virtue of its gravity, radiation pressure, and ionization. The ionization and dynamics of the stellar wind can be probed by analyzing the variability of the X-ray and UV spectra of these systems on timescales ranging from days (e.g., orbital periods) to minutes or less (due to pulsation and intrinsic variability of the X-ray source). Here we review what has been learned about Vela X-1 and can be learned about other such systems from simultaneous observations with HST and X-ray satellites.
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