Toward the Universal Theory of Strings
  We show that the $N=2$ superstrings may be viewed as a special class of the $N=4$ superstrings and demonstrate their equivalence. This allows us to realize all known string theories based on linear algebras and with $N<4$ supersymmetries as special choices of the vacua in the $N=4$ superstring. 
Fiorenzo Bastianelli, Nobuyoshi Ohta and Jens Lyng Petersen