On the Integrability of Covariant Field Theory for Open N=2 Strings
  We discuss the integrability of the Berkovits-Siegel open string field equations and derive an infinite set of their non-local (solution-generating) symmetries. The string field equations are embedded in an infinite system of overdetermined equations (BS hierarchy) associated with hidden string symmetries. The latter enforce the vanishing of most scattering amplitudes for the open N=2 string. 
Olaf Lechtenfeld, Alexander D. Popov