Relative population numbers for the ground states of silicon (left) and magnesium (right) vs. the distance from the donor star in two hydrodynamically consistent PoWR models describing the wind of HD 77581. The dotted lines refer to a model without X-ray illumination, while the simple lines denote the ionization stratification for a model with an X-ray illumination comparable to the situation in Vela X-1; same color represents the same ionization stage of an element. The vertical line denoted with d_ns marks the location of the neutron star.