He II (λ 4686) emission has been detected in the spectrum of the binary Be star ɸ Persei. The He II emission has velocity variations which are 180° out of phase with the velocity variations of the primary. The primary velocity curve is determined from the broad He I and H lines, and the Fe II emission lines. The amplitude of the primary velocity curve (K1) is 16 ± 2 km s−1. The amplitude of the He II velocity curve is 121 ± 9 km s−1.

If one assumes that the He II emission arises from the secondary, minimum masses for the primary and secondary are 29 ± 14 M⊙ and 3.8 ± 1.7 M⊙, respectively. We suggest that the secondary is a hot compact star, either the remaining helium core of a once more massive star, or a neutron star. The primary is too cool, Bl IIIe, to have any appreciable He II emission.