Exotic Branes in String Theory

TL;DR

This work systematically develops exotic branes as ubiquitous, non-geometric objects in string theory, whose charges are encoded by $U$-duality monodromies rather than conventional homology. It connects codimension-2 exotic branes to non-geometric $U$-folds via 10D/11D lifts, and clarifies charge conservation through Page charges that transform covariantly under dualities. The authors construct explicit supergravity backgrounds, notably the $5^2_2$ solution, and demonstrate the exotic supertube mechanism that generates such charges from ordinary branes, producing non-geometric microstates potentially relevant for black-hole physics. The analysis suggests that non-geometric branes are not exceptions but generic ingredients in string theory, with implications for flux compactifications, F-theory generalizations, and the microstate structure of black holes, inviting broader explorations in doubled geometry and $E_{11}$ frameworks.

Abstract

Besides the familiar D-branes, string theory contains a vast number of other non-perturbative objects. While a complete classification is lacking, many of these objects are related to each other through various dualities. Codimension two objects play a special role, because their charges are no longer additive but are instead expressed in terms of holonomies of scalar fields, which is given by an element of the relevant duality group. In this paper we present a detailed exposition of these "exotic" objects, the charges they carry, and their connection to non-geometric compactifications. Despite the name "exotic branes," these objects are in fact ubiquitous in string theory, as they can automatically appear when describing bound states of conventional branes, and as such may be of particular importance in describing the microscopic degrees of freedom of black holes.

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