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. 
I. Pesando