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 explicite solution of the boundary conditions yields effective theory which is symmetric under world-sheet parity transformation $\mathrm{\Omega }:\sigma \to -\sigma$. We recognize effective theory as closed type I superstring theory. Its background fields,beside known $\mathrm{\Omega }$ even fields of the initial IIB theory, contain improvements quadratic in $\mathrm{\Omega }$ odd ones.