D-branes in Nongeometric Backgrounds

TL;DR

This paper reviews Hull's doubled torus formalism to understand D-branes in nongeometric T-fold backgrounds, constraining their spectrum and exploring the moduli space, which remains geometric despite nongeometric features.

Contribution

It applies the doubled torus formalism to analyze D-branes in T-fold backgrounds, providing constraints on their spectrum and insights into their moduli space.

Findings

D-brane spectrum constrained by O(n,n;Z) monodromy

Moduli space of branes is always geometric

D-geometry involves T^n fibrations over multiple covers of the base

Abstract

"T-fold" backgrounds are generically-nongeometric compactifications of string theory, described by T^n fibrations over a base N with transition functions in the perturbative T-duality group. We review Hull's doubled torus formalism, which geometrizes these backgrounds, and use the formalism to constrain the D-brane spectrum (to leading order in g_s and alpha') on T^n fibrations over S^1 with O(n,n;Z) monodromy. We also discuss the (approximate) moduli space of such branes and argue that it is always geometric. For a D-brane located at a point on the base N, the classical ``D-geometry'' is a T^n fibration over a multiple cover of N.