¹ Institut für Astronomie und Astrophysik und Universitäts-Sternwarte München, Scheinerstr. 1, D-81679 München, Germany
² Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165 Ondrejov
³ European Southern Observatory, Karl-Schwarzschildstr. 2, D-85745 Garching, Germany

We model FeII 5317 emission lines and phase-resolved HeI 6678 and 5876 emission lines of the bright B2e+sdO shell binary  Per to find the size and shape of the excitation region inside the circumprimary disk. We find the FeII 5317 emission to originate within 9 stellar radii in an axisymmetric disk around the primary. Orbital phase variations of HeI 6678 are fit in terms of a disk sector with disk radius of 10 stellar radii and opening angle of 120° facing the secondary. This region can be alternatively described by an intersection of a sphere around the secondary and the circumprimary disk with a penetration depth of about 7 R_*. Similar fit values are found for HeI 5876. The enigmatic orbital phase precedence of shell occurrence in the HeI emission features is discussed. We favor a model in which the inner HeI shell is deformed because of differential rotation in combination with a finite recombination time.

Accepted by Astronomy and Astrophysics