TYPE IIB ORIENTIFOLDS WITH NS-NS ANTISYMMETRIC TENSOR BACKGROUNDS - ART. NO. 086001

We consider six dimensional N=1 space-time supersymmetric type IIB ori entifolds with a nonzero untwisted NS-NS sector B field. The B field i s quantized due to the requirement that the type IIB spectrum be left- right symmetric. The presence of the B field results in rank reduction of both 99 and 55 open string sector gauge groups. We point out that in some of the models with a nonzero B field there are extra tensor mu ltiplets in the Z(2) twisted closed string sector, and we explain thei r origin in a simple example. Also, the 59 open suing sector states co me with a multiplicity that depends on the B field. These two facts ar e in accord with anomaly cancellation requirements. We point out relat ions between various orientifolds with and without the B field, and al so discuss the F-theory duals of these models. [S0556-2821(98)05116-9] .