  The usefulness of the $B_s^0 \to D_s^-a_1^+, D_s^- \to \phi \pi^- \to
K^+K^-\pi^-, a_1^+ \to \rho^0 \pi^+ \to \pi^+ \pi^- \pi^+$ decay chain is
investigated for the $B_s$-reconstruction in the future ATLAS $B_s$-mixing
experiment. It is shown that this decay channel is almost as suitable for this
purpose as previously studied $B_s^0 \to D_s^-\pi^+$.
