  We report on a non-perturbative study of the scale-dependent renormalization
factors of a multiplicatively renormalizable basis of Delta B=2 parity-odd
four-fermion operators in quenched lattice QCD. We also present some
preliminary results of the matrix elements related to the mixing parameter of
the B_s-meson. In our lattice formulation, the heavy quark is treated in the
static approximation, while the strange one belongs to a doublet of twisted
mass fermions at full twist, i.e. with twist angle alpha=pi/2. In this
framework, the parity-even Delta B=2 four-fermion operators responsible for the
mixing are rotated onto a linear combination of parity-odd operators in the
above-mentioned basis. Their physical matrix elements between static B_s-mesons
are extracted from lattice correlators with Schroedinger functional boundary
conditions. We observe a suppression of excited state contributions to the
B_{B_s} mixing parameter and speculate about possible explanations.
