D-brane probes, RR tadpole cancellation and K-theory charge

We study RR charge cancellation consistency conditions in string compactifi cations with open string sectors, by introducing D-brane probes in the conf iguration. We show that uncancelled charges manifest as chiral gauge anomal ies in the world-volume of suitable probes. RR tadpole cancellation can the refore be described as the consistency of the effective compactified theory not just in the vacuum, but also in all topological sectors (presence of D -brane probes). The result explains why tadpole cancellation is usually muc h stronger than anomaly cancellation of the compactified theory (in the vac uum sector). We use the probe criterion to construct consistent six-dimensi onal orientifolds of curved K3 spaces, where usual CFT techniques to comput e tadpoles are not valid. As a last application, we consider compactificati ons where standard RR charge cancels but full K-theory charge does not. We show the inconsistency of such models manifests as a global gauge anomaly o n the world-volume of suitable probes. (C) 2001 Elsevier Science B.V. All r ights reserved.