Sketch of the basic ionisation structure for the symbiotic binary EG And in the orbital plane. The shape of the H⁰/H⁺ boundary and isodensities (dotted lines) are shown here for another quiet eclipsing symbiotic star SY Mus (see Shagatova & Skopal 2017; Shagatova 2017) because of the lack of measured H⁰ column densities at ingress phases for EG And. The lines of sight point to the centre of mass of the binary star (black point). Isodensities demonstrate the asymmetric density distribution of the giant’s wind in the orbital plane. In particular, a higher density gradient in front of the RG in the direction of its orbital motion than behind it (Sect. 4.1).