Towards a Complete Twistorization of the Heterotic String
  In $D=3,4,6$ and 10 space--time dimensions considered is a string model invariant under transformations of $N=1$ space--time supersymmetry and $n=D-2$ local worldsheet supersymmetry with the both Virasoro constraints solved in the twistor form. The twistor solution survives in a modified form even in the presence of the heterotic fermions. 
Igor A. Bandos, Martin Cederwall, Dmitrij P. Sorokin and Dmitrij V.   Volkov