Type I superstring theory

Type I string theory is one of the five versions of supersymmetric string theory that exist in 10 spacetime dimensions. It is the only version where the strings are unoriented. Both string orientations are equivalent. Type I superstring theory includes both open and closed strings. 

Ferdinand Gliozzi, Joel Scherk and David Olive paved the way in 1976 for behind the rules for string spectra where only closed strings are present. The original discussion involved type I superstring theory, however, did not lead to models with open strings. 

Augusto Sagnotti, in 1988, proposed that type I string theory can be obtained as an orientifold of type IIB string theory. 32 half D9-branes are added into the vacuum to cancel some anomalies. 

When speaking of low energies, type I superstring theory is described by N=1 supergravity. This is in 10 dimensions and coupled to SO(32) supersymmetric Yang-Mills theory. 

In 1984, Michael Green and John H. Schwarz discover that anomalies in type I superstring theory cancel. This sparked the first superstring revolution. Augusto Sagnotti showed in 1992 that the Green-Schwarz mechanism takes a more general form. It involves several two forms in the cancellation mechanism. 

The relationship between type I superstring theory and the type IIB theory has a large number of consequences. This is both in 10 and in lower dimensions. This was first displayed in the early 1990s  by the String Theory Group at the University of Rome Tor Vergata. This relationship opened the way for construction of entirely new classes of string spectra. This was with or without supersymmetry. 

S-duality

Edward Witten, in the 1990s, proposed that the type I superstring theory with the coupling constant "g" is equivalent to the heterotic SO(32) superstring theory with the coupling constant "1/g."