Gliozzi-Scherk-Olive defects: IIA/IIB walls and the surprisingly stable R7-brane

The recently proposed swampland cobordism conjecture predicts the existence of new nonsupersymmetric 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 Gliozzi-Scherk-Olive projection on the string worldsheet. These include the predicted domain wall between type IIA and IIB superstring theories and the newly discovered R7-brane. We study these defects in two different ways: via long-string probes and target-space effective field theory. We find that the R7-brane can be identified with a collapsed cylindrical configuration of the IIA/IIB wall, and further, that the R7-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 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 so ( 32 ) S-duals of known non-BPS 7- and 8-branes in type I string theory.