We discuss SU(5) Grand Unified Theories in the context of orientifoldcompactifications. Specifically, we investigate two and three D-brane stackrealizations of the Georgi-Glashow and the flipped SU(5) model and analyze themwith respect to their Yukawa couplings. As pointed out in arXiv:0909.0271 themost economical Georgi-Glashow realization based on two stacks genericallysuffers from a disastrous large proton decay rate. We show that allowing for anadditional U(1) D-brane stack this as well as other phenomenological problemscan be resolved. We exemplify with globally consistent Georgi-Glashow modelsbased on RCFT that these D-brane quivers can be indeed embedded in a globalsetting. These globally consistent realizations admit rigid O(1) instantonsinducing the perturbatively missing coupling 10105^H. Finally we show thatflipped SU(5) D-brane realizations even with multiple U(1) D-brane stacks areplagued by severe phenomenological drawbacks which generically cannot beovercome.
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