1. Generalized Veneziano model with isospin;Paton;Nucl. Phys. B,1969
2. Gauge groups for type I superstrings;Schwarz,1982
3. Tree-level constraints on gauge groups for type I superstrings;Marcus;Phys. Lett. B,1982
4. M.R. Gaberdiel and B. Zwiebach, Tensor constructions of open string theories I: Foundations, hep-th/9705038, to appear in Nucl. Phys. B.
5. On the systematics of open-string theories;Bianchi;Phys. Lett. B,1990