D-branes and textures

We examine the flavor structure of the trilinear superpotential couplings w hich can result from embedding the Standard Model within D-brane sectors in type-IIB orientifold models, which are examples within the type-I string f ramework. We find in general that the allowed flavor structures of the Yuka wa coupling matrices to leading order are given by basic variations on the "democratic" texture ansatz. In certain interesting cases, the Yukawa coupl ings have a novel structure in which a single right-handed fermion couples democratically at leading order to three left-handed fermions. We discuss t he viability of such a "single right-handed democracy" in detail; remarkabl y, even though there are large mixing angles in the u, d sectors separately , the CKM mixing angles are small. The analysis demonstrates the ways in wh ich the type-I superstring framework can provide a rich setting for investi gating novel resolutions to the flavor puzzle.