Sketch of the ionization structure of AX Per during the quiescent phase (left, pole-on view) and around the hot star during the active phase (right, edge-on view). During quiescence, the solid thick line represents the H⁰/H⁺ boundary in the steady-state approximation. Including the wind from the hot star and rotation of the binary (denoted by the arrow), a denser region evolves at the preceding front of the hot star, which is further twisted to the front of the giant against its motion (crosses, adapted from Fig. 5 of Bisikalo et al. 2006). The right panel shows a sketch of the ionization structure of the hot component during activity. The neutral zone has the form of a flared disk, whose optically thick rim represents the warm pseudophotosphere. The enhanced wind from the hot star is ionized by its radiation. The He⁺⁺ boundary is denoted by the gray dashed line (see Sect. 4).