Type I

A theory of open and closed unoriented strings in D = 10. The closed string spectrum is a truncation of the spectrum of the type IIB superstring. The projection is implemented by the action of the worldsheet parity operator Ω that exchanges left- and right-movers. The open string spectrum contains at the massless level vector supermultiplets in the 496 adjoint representation of the SO(32) Chan-Paton group. Non-supersymmetric and tachyonic theories of the same kind are also possible in D = 10.

The low-energy effective field theory is in general anomalous but, once the conditions for the vanishing of the tadpolesof massless Ramond-Ramond fields belonging to sectors with nonvanishing Witten index are imposed, the anomaly factorizes and can be disposed of by a generalization of the Green-Schwarz mechanism. Vacuum configurations with open and unoriented superstrings admit an interpretation as open-string descendants of oriented closed string theories, invariant under the...