Anomaly cancellation in K3 orientifolds

We study in detail the pattern of anomaly cancellation in D = 6 Type IIB Z( N) orientifolds, occurring through a generalized Green-Schwarz mechanism in volving several RR antisymmetric tensors and scalars fields. The starting p oint is a direct string theory computation of the inflow of anomaly arising from magnetic interaction of D-branes, O-planes and fixed points, which ar e encoded in topological one-loop partition functions in the RR odd spin-st ructure. All the RR anomalous couplings of these objects are then obtained by factorization. They are responsible for a spontaneous breaking of U(1) a ctors through a Higgs mechanism involving the corresponding hypermultiplets , Some of them are also related by supersymmetry to gauge couplings involvi ng the NSNS scalars sitting in the tensor multiplets. We also comment on th e possible occurrence of tensionless strings when these couplings diverge. (C) 2000 Elsevier Science B.V. All rights reserved.