(Non-)BPS bound states and D-brane instantons

We study superpotential corrections due to decaying BPS D-brane instantons in four-dimensional N = 1 supersymmetric orientifold compactifications. The relevant objects are certain D-brane instantons which are not invariant under the orientifold projection. We show that they can yield superpotential contributions via a multi-instanton process on the BPS locus. This configuration decays in other regions of moduli space. The quasiinstanton associated with the non-BPS decay products has to yield the same superpotential corrections as the former BPS objects. Thus the superpotential may receive corrections from a much larger class of instantons than usually taken into account. We study this concretely for D2-brane instantons along decaying special Lagrangians in Type IIA and for D5-branes instantons carrying holomorphic bundles in Type I theory.