Purely twistorial string with canonical twistor field quantization
  We introduce new purely twistorial scale-invariant action describing the composite bosonic D=4 Nambu-Goto string with target space parametrized by the pair of D=4 twistors. We show that by suitable gauge fixing of local scaling one gets the bilinear twistorial action and canonical quantization rules for the two-dimensional twistor-string fields. We consider the Poisson brackets of all constraints characterizing our model and we obtain four first class constraints describing two Virasoro constraints and two U(1)xU(1) Kac-Moody (KM) local phase transformations. 
Sergey Fedoruk, Jerzy Lukierski