Mixed (open/closed) N=(2,2) string theory as an integrable deformation   of self-duality
  The exact effective field equations of motion, corresponding to the perturbative mixed theory of open and closed (2,2) world-sheet supersymmetric strings, are investigated. It is shown that they are only integrable in the case of an abelian gauge group. The gravitational equations are then stationary with respect to the Born-Infeld-type effective action. 
Sergei V. Ketov (ITP, University of Hannover)