A Hierarchy of Superstrings
  We construct a hierarchy of supersymmetric string theories by showing that the general N-extended superstrings may be viewed as a special class of the (N+1)-extended superstrings. As a side result, we find a twisted (N+2) superconformal algebra realized in the N-extended string. 
Fiorenzo Bastianelli, Nobuyoshi Ohta and Jens Lyng Petersen