Four Dimensional Superstring, Supergravity and Duality

Montonen and Olive's conjecture in 1977 that some theories possess a duality symmetry that interchanges the electrically charged particle with that of negatively charged t' Hooft-Polyakov monopoles which relates strong coupling to weak coupling. This will be nearly the same as Shapere et al, who showed that the equation of motion of the coupled Einstein-Maxwell-axion dilaton system, occurs in low energy in String theory, is invariant under electromagnetic duality transformation; that also interchanges the strong and weak coupling limit of the theory. These ideas have been persued in the $SO(6)\otimes SO(5)$ gauge groups as well as in the supersymmetric group of a four dimensional String. We analyse and find that they give clearer evidence of string theory in two and four dimensional supergravity theory. However, the application to supergravity is beset with the normal difficulties and suggestions have been made how to settle them.