D-strings in unconventional type I vacuum configurations

TL;DR

The paper analyzes D-string bound states in unconventional Type I vacua and their duals, showing exact BPS-spectrum matching with heterotic CHL models and Type II (4,0) duals without open strings. It develops D-string worldvolume theories under orbifold/orientifold actions, computes elliptic genera for symmetric-product CFTs, and demonstrates that long-string sectors reproduce CHL windings and dual-type spectra. It then studies moduli-dependent, BPS-saturated F^4 threshold corrections, showing Type I D-string instantons reproduce the moduli dependence predicted by Type II duals and establishing one-loop exactness on the Type IIB side. Together, these results reinforce the web of dualities among unconventional Type IIB orientifolds, Type I without open strings, and CHL heterotic models, and highlight a rich non-perturbative structure in higher-dimensional 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.