Tensionless strings, correspondence with SO(D,D) sigma model
  String theory with perimeter action is tensionless by its geometrical nature and has pure massless spectrum of higher spin gauge particles. I demonstrate that liner transformation of the world-sheet fields defines a map to the SO(D,D) sigma model equipped by additional Abelian constraint, which breaks SO(D,D) to a diagonal SO(1,D-1). The effective tension is equal to the square of the dimensional coupling constant of the perimeter action. This correspondence allows to view the perimeter action as a "square root" of the Nambu-Goto area action. The aforementioned map between tensionless strings and SO(D,D) sigma model allows to introduce the vertex operators in full analogy with the standard string theory and to confirm the form of the vertex operators introduced earlier. 
G.Savvidy