D-branes and textures

We examine the flavor structure of the trilinear superpotential couplingswhich can result from embedding the Standard Model within D brane sectors inType IIB orientifold models, which are examples within the Type I stringframework. We find in general that the allowed flavor structures of the Yukawacoupling matrices to leading order are given by basic variations on the"democratic" texture ansatz. In certain interesting cases, the Yukawa couplingshave a novel structure in which a single right-handed fermion couplesdemocratically at leading order to three left-handed fermions. We discuss theviability of such a ``single right-handed democracy'' in detail; remarkably,even though there are large mixing angles in the u,d sectors separately, theCKM mixing angles are small. The analysis demonstrates the ways in which theType I superstring framework can provide a rich setting for investigating novelresolutions to the flavor puzzle.Comment: 23 pages, references adde