The KK-Monopole/NS5-Brane in Doubled Geometry

The Kaluza-Klein monopole has been recognized as a string background with significant non-geometric features: it must appear “localized” to winding strings to match the NS5-brane’s localization on the T-dual circle. In this work, we explicitly construct this T-dual system in the doubled geometry formalism, which proves to successfully describe the duality despite a broken isometry on one side of the duality pair. We further suggest an extension of the doubled formalism to the gauged linear sigma models describing this system (both bosonic and supersymmetric) and show that previous calculations of worldsheet instanton effects are best understood in this doubled form.