Hidden symmetries of the open N=2 string

It is known for ten years that self-dual Yang-Miffs theory is the effective field theory of the open N = 2 string in (2 + 2)-dimensional space-time. W e uncover an infinite set of Abelian rigid string symmetries, corresponding to the symmetries and integrable hierarchy of the self-dual Yang-Mills equ ations. The twistor description of the latter naturally connects with the B RST approach to string quantization, providing an interpretation of the pic ture phenomenon in terms of the moduli space of string backgrounds.