Orientifolds, hypercharge embeddings and the Standard Model
  The embedding of the SM hypercharge into an orientifold gauge group is studied. Possible embeddings are classified, and a systematic construction of bottom-up configurations and top-down orientifold vacua is achieved, solving the tadpole conditions in the context of Gepner orientifolds. Some hypercharge embeddings are strongly preferred compared to others. Configurations with chiral antisymmetric tensors are suppressed. We find among others, genuine examples of supersymmetric SU(5), flipped SU(5), Pati-Salam and trinification vacua with no chiral exotics. 
P. Anastasopoulos (Athens U.), T.P.T. Dijkstra (NIKHEF, Amsterdam), E.   Kiritsis (Crete U. & Ecole Polytechnique, CPHT), A....