  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
$\Omega:\sigma\to-\sigma$. We recognize effective theory as closed type I
superstring theory. Its background fields,beside known $\Omega$ even fields of
the initial IIB theory, contain improvements quadratic in $\Omega$ odd ones.
