In the mid 1990s, physicists noticed that these five string theories are actually related by highly nontrivial dualities. One of these dualities is S-duality. The existence of S-duality in string theory was first proposed by Ashoke Sen in 1994.Sen 1994 It was shown that type IIB string theory with the coupling constant g is equivalent via S-duality to the same string theory with the coupling constant 1/g. Similarly, type I string theory with the coupling g is equivalent to the SO(32) heterotic string theory with the coupling constant 1/g.
