Fricke S-duality in CHL models

Journal article, 2015

We consider four dimensional CHL models with sixteen spacetime supersymmetries obtained from orbifolds of type IIA superstring on K3×T 2 by a ℤ𝑁 symmetry acting (possibly) non-geometrically on K3. We show that most of these models (in particular, for geometric symmetries) are self-dual under a weak-strong duality acting on the heterotic axio-dilaton modulus S by a “Fricke involution” S → −1/NS. This is a novel symmetry of CHL models that lies outside of the standard SL(2,ℤ)-symmetry of the parent theory, heterotic strings on T 6. For self-dual models this implies that the lattice of purely electric charges is N-modular, i.e. isometric to its dual up to a rescaling of its quadratic form by N. We verify this prediction by determining the lattices of electric and magnetic charges in all relevant examples. We also calculate certain BPS-saturated couplings and verify that they are invariant under the Fricke S-duality. For CHL models that are not self-dual, the strong coupling limit is dual to type IIA compactified on 𝑇6/ℤ𝑁, for some ℤ𝑁-symmetry preserving half of the spacetime supersymmetries.