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We study the conditions under which D-brane instantons in Type II orientifoldcompactifications generate a non-perturbative superpotential. If the instantonis non-invariant under the orientifold action, it carries four instead of thetwo Goldstone fermions required for superpotential contributions. Unless theseare lifted, the instanton can at best generate higher fermionic F-terms ofBeasley-Witten type. We analyse two strategies to lift the additional zeromodes. First we discuss the process of instantonic brane recombination in TypeIIA orientifolds. We show that in some cases charge invariance of the measureenforces the presence of further zero modes which, unlike the Goldstinos,cannot be absorbed. In other cases, the instanton exhibits reparameterisationzero modes after recombination and a superpotential is generated if these arelifted by suitable closed or open string couplings. In the second part of thepaper we address lifting the extra Goldstinos of D3-brane instantons bysupersymmetric three-form background fluxes in Type IIB orientifolds. Thisrequires non-trivial gauge flux on the instanton. Only if the part of thefermionic action linear in the gauge flux survives the orientifold projectioncan the extra Goldstinos be lifted.

Lifting D-Instanton Zero Modes by Recombination and Background Fluxes