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

Jonathan J. Heckman1,2,*, Jacob McNamara3,†, Julio Parra-Martinez4,‡, and Ethan Torres5,§

  • *Contact author: jheckman@sas.upenn.edu
  • †Contact author: jmcnamar@caltech.edu
  • ‡Contact author: julio@ihes.fr
  • §Contact author: ethan.martin.torres@cern.ch
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Phys. Rev. D 113, 066021 – Published 30 March, 2026

DOI: https://doi.org/10.1103/wqys-zsyf

Abstract

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 Z2 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.

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Phys. Rev. D 113, 066021– Published 30 March, 2026
  • Received 12 January 2026
  • Accepted 11 March 2026
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