The string measure and spectral flow of critical N = 2 strings

The general structure of N = 2 moduli space at arbitrary genus and instanton number is investigated. The N = 2 NSR string measure is calculated, yielding picture- and U(1) ghost number-changing operator insertions. An explicit formula for the spectral flow operator acting on vertex operators is given, and its effect on N = 2 string amplitudes is discussed.