Stringy instanton effects in models with rigid magnetised D-branes

We compute instantonic effects in globally consistent T-6/Z(2) x Z(2) orientifold models with discrete torsion and magnetised D-branes. We consider fractional branes and instantons wrapping the same rigid cycles. We clarify and analyse in detail the low-energy effective action on D-branes in these models. We provide explicit examples where instantons induce linear terms in the charged fields. or non-perturbative mass terms are generated. We also find examples where the gauge theory on fractional branes has conformal symmetry at one-loop, broken by instantonic mass terms at a hierarchically small energy scale. (C) 2009 Elsevier B.V. All rights reserved.