  We report evidence for the $b\to u$ transition $B^0 \to D_s^{+}\pi^-$ and the
results of a search for $B^0 \to D_s^{*+} \pi^-$ from a sample of $61.6\times
10^6$ $Y(4S)$ decays into $B$ meson pairs collected with the BaBar detector at
the PEP II asymmetric $e^{+}e^{-}$ collider.
  The observed $B^0 \to D_s^{+} \pi^-$ yield has a probability of $4.4 \times
10^{-4}$ to be a fluctuation of the background (3.5 $\sigma$) and we measure
the branching fraction $B^0 \to D_s^{+} \pi^-=3.1 \pm 1.0 ({\rm stat.})\pm 1.0
({\rm syst.})$.
  We also set a limit $B^0 \to D_s^{*+} \pi^- <~4.3\times 10^{-5}$ at 90% C.L.
