  The stellar-wind structure in high-mass X-ray binaries (HMXBs) is
investigated through modelling of their ultraviolet (UV) resonance lines. For
the OB supergiants in two systems, Vela X-1 and 4U1700-37, high-resolution UV
spectra are available; for Cyg X-1, SMC X-1, and LMC X-4 low-resolution spectra
are used. In order to account for the non-monotonic velocity structure of the
stellar wind, a modified version of the Sobolev Exact Integration (SEI) method
by Lamers et al. (1987) is applied. The orbital modulation of the UV resonance
lines provides information on the size of the Stroemgren zone surrounding the
X-ray source. The amplitude of the observed orbital modulation (known as the
Hatchett-McCray effect), however, also depends on the density- and velocity
structure of the ambient wind. Model profiles are presented that illustrate the
effect on the appearance of the HM effect by varying stellar-wind parameters.
The q parameter of Hatchett & McCray (1977), as well as other parameters
describing the supergiant's wind structure, are derived for the 5 systems. The
X-ray luminosity needed to create the observed size of the Stroemgren zone is
consistent with the observed X-ray flux. The derived wind parameters are
compared to those determined in single OB supergiants of similar spectral type.
Our models naturally explain the observed absence of the HM effect in
4U1700-37. The orbital modulation in Vela X-1 indicates that besides the
Stroemgren zone other structures are present in the stellar wind (such as a
photo-ionization wake). The ratio of the wind velocity and the escape velocity
is found to be lower in OB supergiants in HMXBs than in single OB supergiants
of the same effective temperature.
