  Periodicities observed in two Fast Radio Burst (FRB) sources (16 days in FRB
180916.J0158+65 and 160 days in FRB 121102) are consistent with that of tight,
stellar mass binary systems. In the case of FRB 180916.J0158+65 the primary is
an early OB-type star with mass loss rate $\dot{M} \sim 10^{-8}- 10^{-7}
M_\odot$ yr$^{-1}$, and the secondary a neutron star. The observed periodicity
is not intrinsic to the FRB's source, but is due to the orbital phase-dependent
modulation of the absorption conditions in the massive star's wind. The
observed relatively narrow FRB activity window implies that the primary's wind
dynamically dominates that of the pulsar, $\eta = L_{sd}/(\dot{M} v_w c) \leq
1$, where $L_{sd} $ is pulsar spin-down, $\dot{M}$ is the primary's wind mass
loss rate and $v_w$ is its velocity. The condition $\eta \leq 1$ requires
mildly powerful pulsar with $L_{sd} \lesssim 10^{37}$ erg $s^{-1}$. The
observations are consistent with magnetically-powered radio emission
originating in the magnetospheres of strongly magnetized neutron stars, the
classical magnetars.
