On the complementarity of F-theory, orientifolds, and heterotic strings Original Research Article

We study F-theory duals of six-dimensional heterotic vacua in extreme regions of moduli space where the heterotic string is very strongly coupled. We demonstrate how to use orientifold limits of these F-theory duals to regain a perturbative string description. As an example, we reproduce the spectrum of a T4/Z4 orientifold as an F-theory vacuum with a singular K3 fibration. We relate this vacuum to previously studied heterotic E8 × E8 compactifications on K3.