On Aspects and Implications of the New Covariant 4D, N = 1 Green-Schwarz   Sigma-model Action
  Utilizing (2,0) superfields, we write a supersymmetry-squared action and partially relate it to the new formulation of the Green-Schwarz action given by Berkovits and Siegel. Recent results derived from this new formulation are discussed within the context of some prior proposals in the literature. Among these, we note that 4D, N = 1 beta-FFC superspace geometry with a composite connection for R-symmetry has now been confirmed as the only presently known limit of 4D, N = 1 heterotic string theory that is derivable in a completely rigorous manner. 
S.J.Gates Jr