Internal magnetic fields and supersymmetry in orientifolds

Within the context of type I strings, we show the equivalence between BPS D9 branes with internal magnetic fluxes Hi in the three torii and non-BPS D3 branes with inverted internal magnetic fluxes 1/Hi. We then construct new supersymmetric examples of Z2 × Z2 orientifolds with discrete torsion which in the past had only non-supersymmetric solutions and emphasize the role of new twisted tadpole cancellation conditions, arising in the presence of magnetic fields, in order to get a consistent spectrum. In a second and independent part of the paper, we construct a new nine-dimensional type IIB orientifold with Scherk-Schwarz deformation which has the peculiarity of introducing a new type of non-BPS O9 planes and which contains as top branes a Scherk-Schwarz deformation of non-BPS D9 branes. The model contains charged D7 and D3 branes with a soft supersymmetry breaking spectrum. Unité mixte du CNRS et de l’EP, UMR 7644. Unité mixte du CNRS, UMR 8627.