String interactions in a plane-fronted parallel-wave spacetime

We argue that string interactions in a plane-fronted parallel-wave spacetime are governed by an effective coupling g(eff) = g(s)(mup(+)alpha')f(mup(+)alpha') where f(mup(+)alpha') is proportional to the light-cone energy of the string states involved in the interaction. This simply follows from generalities of a matrix string description of this background. g(eff) nicely interpolates between the expected result (g(s)) for flat space (small mup(+)alpha') and a recently conjectured expression from the perturbative gauge theory side (large mup(+)alpha').