Relations between the orbital period (P_orb) and the spin period (P_spin) for IPs (black; Mukai 2017) and Be/NS binaries (gray; Reig 2011). For the latter, a larger symbol size is used for the persistent (X Per-type) sources than for the others. The phenomenological linear relation is shown with dashed lines for each class. The relation for Be/WD binaries scaled from the one for the Be/NS binaries (Apparao 2002) is shown with the red dashed line. P_orb is shown for γ Cas (Harmanec et al. 2000) with the red dotted vertical line. (Color online)