Type I background fields in terms of type IIB ones

We choose such boundary conditions for open IIB superstring theory which preserve N=1 SUSY. The explicit solution of the boundary conditions yields effective theory which is symmetric under world-sheet parity transformation Ω:σ→−σ. We recognize effective theory as closed type I superstring theory. Its background fields, beside known Ω even fields of the initial IIB theory, contain improvements quadratic in Ω odd ones.