  Starting from the type IIB string on the Z orbifold, we construct some chiral
open-string vacua with N=1 supersymmetry in four dimensions. The Chan-Paton
group depends on the (quantized) NS-NS antisymmetric tensor. The largest
choice, SO(8)xSU(12)xU(1), has an anomalous U(1) factor whose gauge boson
acquires a mass of the order of the string scale. The corresponding open-string
spectrum comprises only Neumann strings and includes three families of chiral
multiplets in the (8,12*) + (1,66) representation. A comparison is drawn with a
heterotic vacuum with non-standard embedding, and some properties of the
low-energy effective field theory are discussed.
