String interactions in gravitational wave backgrounds

The non-compact CFT of a class of NS-supported pp-wave backgrounds is solved exactly. The associated tree-level covariant string scattering amplitudes are calculated. The S-matrix elements are well-defined, dual but not analytic as a function of $p^+$. They have poles corresponding to physical intermediate states with $p^+\not =0$ and logarithmic branch cuts due to on-shell exchange of spectral-flow images of $p^+=0$ states. When $\mu\to 0$ a smooth flat space limit is obtained. The $\mu\to\infty$ limit, unlike the case of RR-supported pp-waves, gives again a flat space theory.