Instanton corrections to the effective action of the CHL string and its type I dual

TL;DR

This paper computes instanton corrections to gauge couplings in CHL string compactifications and verifies their consistency with type I duality, highlighting identical non-perturbative effects across models.

Contribution

It provides the first detailed calculation of instanton effects in CHL string compactifications and tests S-duality by comparing heterotic and type I results.

Findings

Threshold corrections to F^4 terms are computed in heterotic CHL string.

Non-perturbative D-string effects are identified and matched across dual models.

Instanton corrections are shown to be identical in different type I compactifications.

Abstract

In this letter, we investigate corrections to quartic gauge couplings in compactifications of string theories on a 2-torus without vector structure. First, we calculate the threshold corrections to F^4 terms in the heterotic CHL string. Then, using the CHL string/type I duality dictionary, we map these corrections to perturbative and non-perturbative effects in type I string compactified on 2-torus without vector structure. Comparing the perturbative terms provides a quantitative test of the S-duality conjecture. The non-perturbative contributions are due to D-strings wrapping the torus. A T-duality along one of the compactified directions allows us to compare these instanton corrections to the ones obtained in a standard type I compactifications. The most striking feature is that these non-perturbative couplings turn out to be identical for both models.