Instanton induced open string superpotentials and branes at singularities

We study different aspects of the non-perturbative superpotentials induced by Euclidean E3-branes on systems of D3/D7-branes located at Abelian orbifold singularities. We discuss in detail how the induced couplings are consistent with the U(1) symmetries carried by the D3/D7 branes. We construct di ff erent compact and non-compact examples, and show phenomenologically relevant couplings like mu-terms or certain Yukawa couplings generated by these E3 instantons. Some other novel e ff ects are described. We show an example where E3 instantons combine with standard gauge instantons to yield new multi-instanton e ff ects contributing to superpotential, along the lines of ref. [28]. In the case of non-SUSY Z N tachyon-free singularities it is shown how E3-instantons give rise to non perturbative scalar couplings including exponentially suppressed scalar bilinears.