Covariant N=2 heterotic string in four dimensions
  We construct a covariant formulation of the heterotic superstring on K3 times T^2 with manifest N=2 supersymmetry. We show how projective superspace appears naturally in the hybrid formulation giving a (partially) geometric interpretation of the harmonic parameter. The low-energy effective action for this theory is given by a non-standard form of N=2 supergravity which is intimately related to the N=1 old-minimal formulation. This formalism can be used to derive new descriptions of interacting projective superspace field theories using Berkovits' open string field theory and the the heterotic Berkovits-Okawa-Zwiebach construction. 
William D. Linch III and Brenno Carlini Vallilo