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In string theory, N=2 superstring is a theory in which the worldsheet admits N=2 supersymmetry rather than N=1 supersymmetry as in the usual superstring. The target space (a term used for a generalization of space-time) is four-dimensional, but either none or two of its dimensions are time-like, i.e. it has either 4+0 or 2+2 dimensions. The spectrum consists of only one massless scalar, which describes gravitational fluctuations of self-dual gravity. The target space theory is therefore self-dual gravity, and is thought to consist no local (or propagating) degrees of freedom.