On R4 Terms and MHV Amplitudes in N=5, 6 Supergravity Vacua of Type II Superstrings

We compute one-loop threshold corrections to R-4 terms in N = 5, 6 supergravity vacua of Type II superstrings. We then discuss nonperturbative corrections generated by asymmetric D-brane instantons. Finally we derive generating functions for MHV amplitudes at tree level in N = 5, 6 supergravities.