On the Worldsheet Formulation of the Six-Dimensional Self-Dual String
  Despite recent evidence indicating the existence of a new kind of self-dual six-dimensional superstring, no satisfactory worldsheet formulation of such a string has been proposed. In this note we point out that a theory built from Z_4 parafermions may have the right properties to describe the light-cone conformal field theory of this string. This indicates a possible worldsheet formulation of this theory. 
Philip C. Argyres (Rutgers University) and Keith R. Dienes (Institute   for Advanced Study, Princeton)