  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.
