A comment on discrete Kalb-Ramond field on orientifold and rank reduction

We show that the rank reduction of the gauge group on orientifolds in presence of non-vanishing discrete Kalb-Ramond field can be explained by the presence of an induced field strength in a non-trivial bundle on the branes. This field strength is also necessary for the tadpole cancellation and the number of branes is left unchanged by the presence of the discrete Kalb-Ramond background. We moreover give a derivation of the normalization of the Möbius amplitude in presence of a non-vanishing Kalb-Ramond field from first principles.