Rappaport & Van den Heuvel (1982) suggested the following formation scenario for B emission (Be)/X-ray binaries: the progenitor of the neutron star is the initial primary star with mass M1 = 10-20 M⊙, which during hydrogen-shell burning transfers mass to the companion (with M2 < M1). The secondary is spun up due to disk-accretion (e.g. Packet 1981) and has become a rapidly rotating Be star (M ≃ 10-20 M⊙).
With this scenario the observed orbital periods (Porb > 15 d) of the Be/X-ray binaries can be explained. This was shown by van den Heuvel (1983) by adopting an idealized period distribution for unevolved O- and B-type close binaries with Porb, < 30 d (see Fig. 1) and assuming conservative evolution, i.e. that mass and angular momentum are conserved during mass transfer.