1996A&A...314L..13L

The relation between the spin period P_s and the orbital period P_orb of neutron stars in Be/X-ray binaries is investigated. The spin periods of neutron stars in such systems can be appropriately explained in terms of the equilibrium spin, at which a small torque is exerted on the neutron stars. But the equilibrium is achieved in different ways. In short orbital period Be/X-ray binaries (P_orb≤100days), the equilibrium spin is reached by angular momentum transfer via a disk, which is formed in the equatorial wind of the Be stars. In wider Be/X-ray binaries, the low angular momentum of the wind matter prevents the existence of an accretion disk, and the neutron star's spin remains close to its previous equilibrium value at an earlier evolutionary stage, similar as supergiant/X-ray binaries.