The Covariant Superstring on K3
  We compactify the pure spinor formalism on a K3 surface. The pure spinor splits into a six-dimensional pure spinor, a projective superspace harmonic, and 6 non-covariant variables. A homological algebra argument reduces the calculation of the cohomology of the Berkovits differential to a "small" Hilbert space which is the string-theoretic analogue of projective superspace. The description of the physical state conditions is facilitated by lifting to the full harmonic superspace, which is accomplished by the introduction of the missing harmonics as non-minimal variables. Finally, contact with the hybrid formalism is made by returning to the small Hilbert space and fermionizing the projective parameter. 
Osvaldo Chandia, William D. Linch III, and Brenno Carlini Vallilo
hep-th