New Type II String Theories with Sixteen Supercharges

TL;DR

This work presents two exact, 16-supercharge backgrounds of type IIA string theory: a Wilson line for $(-1)^{F_{L_S}}$ on a circle and an orbifold by reflection enhanced by the same phase. By analyzing closed and open strings in Green-Schwarz and RNS formalisms and lifting the Wilson-line background to M-theory, the authors uncover a self-dual circle with enhanced $SU(2)_L\times U(1)_R$ gauge symmetry and a novel M-theory duality on a Klein bottle. The orbifold background yields a six-dimensional $(0,2)$ theory with twisted moduli, and reveals stable non-BPS branes pinned to the fixed locus, including D1, D3, and D5, with intriguing boundary-state and tadpole structure and a rich type IIB/S-dual interpretation. Together, these models illuminate aspects of highly supersymmetric, potentially nongeometric string backgrounds and provide explicit, solvable arenas to study non-BPS branes and discrete charges in M-theory dualities.

Abstract

We present two new backgrounds of type IIA string theory preserving 16 supercharges. One is a Wilson line for (-1) to the left-moving spacetime fermion number, and the other is an orbifold by a reflection of four coordinates, along with the action of (-1) to the left-moving spacetime fermion number. The Wilson line theory has many new phenomena, including a self-duality of type IIA on a single circle, enhanced gauge symmetry at the self-dual radius, and a T-duality between uncharged and (locally) charged branes. The orbifold theory also presents many novel features, including charged, stable non-BPS D1, D3, and D5-branes pinned to the fixed locus, and an instability of the D0-brane near the fixed locus.