Asymmetric orientifolds, brane supersymmetry breaking and non-BPS branes

A new class of six-dimensional asymmetric orientifolds is considered where the orientifold operation is combined with T-duality. The models are supers ymmetric in the bulk, but the cancellation of the tadpoles requires the int roduction of brane configurations that break supersymmetry. These can be de scribed by D7-brane-antibrane pairs, non-BPS D8-branes or D9-brane-antibran e pairs. The transition between these different configurations and their st ability is analysed in detail. (C) 2000 Elsevier Science B.V. All rights re served.