GSO Defects: IIA/IIB Walls and the Surprisingly Stable $\mathrm{R}7$-Brane

Abstract: The recently proposed Swampland Cobordism Conjecture predicts the existence of new non-supersymmetric objects which supplement the spectrum of low-energy gravitational effective field theories. In this paper, we study a subset of these defects related to the GSO projection on the string worldsheet. These include the predicted domain wall between Type IIA and IIB superstring theories and the newly-discovered $\mathrm{R}7$-brane. We study these defects in two different ways: via long-string probes and target-space effective field theory. We find that the $\mathrm{R}7$-brane can be identified with a collapsed cylindrical configuration of the IIA/IIB wall, and further, that the $\mathrm{R}7$-brane is stable, in contrast to previous expectations. Moreover, we argue that BPS D-branes pulled across the IIA/IIB wall become non-BPS D-branes, which we identify with fluxbrane configurations. We show that the non-BPS D-branes of either Type II theory are charged under a $\mathbb{Z}_2$ remnant of the Ramond-Ramond potentials of the other, which we identify with the mod 2 reduction of the Ramond-Ramond fluxes. Similar considerations provide a complementary perspective on the Heterotic ${\mathfrak{so}(32)}$ S-duals of known non-BPS 7- and 8-branes in Type I string theory.