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We perform a detailed conformal field theory analysis of D2-brane instantoneffects in four-dimensional type IIA string vacua with intersecting D6-branes.In particular, we explicitly compute instanton induced fermion two-pointcouplings which play the role of perturbatively forbidden Majorana mass termsfor right-handed neutrinos or MSSM mu-terms. These results can readily beextended to higher-dimensional operators. In concrete realizations of suchnon-perturbative effects, the Euclidean D2-brane has to wrap a rigid,supersymmetric cycle with strong constraints on the zero mode structure. Theirimplications for Type IIA compactifications on the T^6/(Z_2 x Z_2) orientifoldwith discrete torsion are analyzed. We also construct a local supersymmetricGUT-like model allowing for a class of Euclidean D2-branes whose fermionic zeromodes meet all the constraints for generating Majorana masses in thephenomenologically allowed regime. Together with perturbatively realized Diracmasses, these non-perturbative couplings give rise to the see-saw mechanism.Comment: 33 pages, 4 figures; v2: minor corrections and clarifications adde

Computation of D-brane instanton induced superpotential couplings: Majorana masses from string theory