The supersymmetric standard model from the Z6 orientifold?

We construct = 1 supersymmetric fractional branes on the Z6 orientifold. Intersecting stacks of such branes are needed to build a supersymmetric standard model. If a, b are the stacks that generate the SU(3)c and SU(2)L gauge particles, then, in order to obtain just the chiral spectrum of the (supersymmetric) standard model (with non-zero Yukawa couplings to the Higgs mutiplets), it is necessary that the number of intersections a b of the stacks a and b, and the number of intersections a b of a with the orientifold image b of b satisfy (a b, a b) = (2, 1) or (1, 2). It is also necessary that there is no matter in symmetric representations of the gauge group. We have found a number of examples having these properties. Different lattices give different solutions and different physics.