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We revisit type I compactifications with a Spin(32)/Z2 gauge bundle thatadmits no vector structure. We elucidate the relation of this Z2 obstruction todiscrete B-field flux and to 't Hooft flux and clarify some subtleties in theT-duality transformation to type IIA intersecting D-brane models. We reexaminethe earliest 3-generation GUT model on magnetized D-branes and show itsconsistency when a discrete B-flux is switched on. We further generalizepartially known results for toroidal models to type I compactifications withoutvector structure and their mirror dual type IIA orientifolds on genuineCalabi-Yau manifolds. We illustrate this by working out the example of thequintic in some detail.

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