Observation of the decays $B^ - \to D_s$ (*) + $K^ - \pi^ - $

We report the observation of decays B0->D_s(*)+pi- and B0->D_s(*)-K+ in a sample of 230*10^6 Upsilon(4S)->BBbar events recorded with the BaBar detector at the PEP-II asymmetric-energy e+e- storage ring. We measure the branching fractions B(B0->D_s+pi-)=(1.3 +/- 0.3 (stat.) +/- 0.2 (syst.))*10^{-5}, B(B0->D_s-K+)=(2.5 +/- 0.4 (stat.) +/- 0.4 (syst.))*10^{-5}, B(B0->D_s*+pi-)=(2.8 +/- 0.6 (stat.) +/- 0.5 (syst.))*10^{-5}, and B(B0->D_s*-K+)=(2.0 +/- 0.5 (stat.) +/- 0.4 (syst.))*10^{-5}. The significance of the measurements to differ from zero are 5, 9, 6, and 5 standard deviations, respectively.