Search for the $W^-$ exchange decays $B^0 \to D_s$ (*) - $D_s$ (*) +

We report a search for the decays B0→D−sD+sB0→Ds−Ds+, B0→D∗−sD+sB0→Ds∗−Ds+, B0→D∗−sD∗+sB0→Ds∗−Ds∗+ in a sample of 232 million Υ(4S)Υ(4S) decays to \BBb ~pairs collected with the \babar detector at the PEP-II asymmetric-energy e+e−e+e− storage ring. We find no significant signal and set upper bounds for the branching fractions: B(B0→D−sD+s)<1.0×10−4,B(B0→D∗−sD+s)<1.3×10−4B(B0→Ds−Ds+)<1.0×10−4,B(B0→Ds∗−Ds+)<1.3×10−4 and B(B0→D∗−sD∗+s)<2.4×10−4B(B0→Ds∗−Ds∗+)<2.4×10−4 at 90% confidence level.