D-branes at singularities, compactification, and hypercharge

We report on progress towards the construction of SM- like gauge theories on the world-volume of D-branes at a Calabi-Yau singularity. In particular, we work out the topological conditions on the embedding of the singularity inside a compact CY threefold, that select hypercharge as the only light U(1) gauge factor. We apply this insight to the proposed open string realization of the SM of hep-th/0508089, based on a D3-brane at a dP(8) singularity, and present a geometric construction of a compact Calabi-Yau threefold with all the required topological properties.