Nonperturbative Yukawa couplings from string instantons

Nonperturbative D-brane instantons can generate perturbatively absent though phenomenologically relevant couplings for type II orientifold compactifications with D-branes. We discuss the generation of the perturbatively vanishing SU(5) GUT Yukawa coupling of type < 10 10 5(H)>. Moreover, for a simple globally consistent intersecting D6-brane model, we discuss the generation of mass terms for matter fields. This can serve as a mechanism for decoupling exotic matter.