  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.
