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We study heterotic E8 × E8 models that are dual to compactifications of Ftheory and type IIA string on certain classes of elliptically fibered Calabi-Yau manifolds. Different choices for the specific torus in the fibration have heterotic duals that are most easily understood in terms of E8 ×E8 models with gauge backgrounds of type H ×U(1)8−d, where H is a(More)
We study different aspects of the construction of D=4, N=1 type IIB orientifolds based on toroidal ZN and ZM×ZN , D=4 orbifolds. We find that tadpole cancellation conditions are in general more constraining than in six dimensions and that the standard Gimon-Polchinski orientifold projection leads to the impossibility of tadpole cancellations in a number of(More)
We consider D = 6, N = 1, ZM orbifold compactifications of heterotic strings in which the usual modular invariance constraints are violated. It is argued that in the presence of non-perturbative effects many of these vacua are nevertheless consistent. The perturbative massless sector can be computed explicitly from the perturbative mass formula subject to(More)
Up to now chiral type IIA vacua have been mostly based on intersecting D6-branes wrapping special Lagrangian 3-cycles on a CY3 manifold. We argue that there are additional BPS Dbranes which have so far been neglected, and which seem to have interesting model-building features. They are coisotropic D8-branes, in the sense of Kapustin and Orlov. The D8-branes(More)
A construction of compact tachyon-free orientifolds of the nonsupersymmetric Type 0B string theory is presented. Moreover, we study effective non-supersymmetric gauge theories arising on self-dual D3-branes in Type 0B orbifolds and orientifolds. PACS numbers: 11.25.-w, 11.25.Sq Submitted to: Class. Quantum Grav. § Talk presented by R.B. at Strings ’99,(More)
We consider 4-dimensional string models obtained by tensoring N=2 coset theories with non-diagonal modular invariants. We present results from a systematic analysis including moddings by discrete symmetries. ∗ Work partially supported by CONICET of Argentina, by the Commission of the European Communities under contract No. C 11-0540-M(TT), and by Fundación(More)
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