D-strings in unconventional type I vacuum configurations

TL;DR

This paper analyzes D-string bound states in various unconventional type I vacuum configurations, confirming dualities with heterotic and type II models through BPS spectrum matching and exploring threshold corrections.

Contribution

It provides a detailed spectrum analysis of D-string states in generalized type I vacua and confirms dualities with heterotic and type II models, extending to K3 compactifications.

Findings

Matching of BPS spectra confirms dualities.

Exact moduli dependence of F^4 terms derived.

D-instanton contributions reproduce threshold corrections.

Abstract

We determine the spectrum of D-string bound states in various classes of generalized type I vacuum configurations with sixteen and eight supercharges. The precise matching of the BPS spectra confirms the duality between unconventional type IIB orientifolds with quantized NS-NS antisymmetric tensor and heterotic CHL models in D=8. A similar analysis puts the duality between type II (4,0) models and type I strings {\it without open strings} on a firmer ground. The analysis can be extended to type II (2,0) asymmetric orbifolds and their type I duals that correspond to unconventional K3 compactifications. Finally we discuss BPS-saturated threshold corrections to the correponding low-energy effective lagrangians. In particular we show how the exact moduli dependence of some F^4 terms in the eight-dimensional type II (4,0) orbifold is reproduced by the infinite sum of D-instanton contributions in the dual type I theory.