  We study locking of the modulation frequency of a relative periodic orbit in
a general $S^1$-equivariant system of ordinary differential equations under an
external forcing of modulated wave type. Our main result describes the shape of
the locking region in the three-dimensional space of the forcing parameters:
intensity, wave frequency, and modulation frequency. The difference of the wave
frequencies of the relative periodic orbit and the forcing is assumed to be
large and differences of modulation frequencies to be small. The intensity of
the forcing is small in the generic case and can be large in the degenerate
case, when the first order averaging vanishes. Applications are external
electrical and/or optical forcing of selfpulsating states of lasers.
