Making the hyper--K\"ahler structure of N=2 quantum string manifest
  We show that the Lorentz covariant formulation of N=2 string in a curved space reveals an explicit hyper--K\"ahler structure. Apart from the metric, the superconformal currents couple to a background two--form. By superconformal symmetry the latter is constrained to be holomorphic and covariantly constant and allows one to construct three complex structures obeying a (pseudo)quaternion algebra. 
S. Bellucci, A.V. Galajinsky, E. Latini