  We note that the two body nonleptonic pure tree decays $B_c^\pm \to D_s^\pm
D^0(\bar D^0)$ and the corresponding vector-vector modes $B_c^\pm \to D_s^{*
\pm} D^{*0}(\bar D^{* 0}) $ are well suited to extract the weak phase $\gamma$
of the unitarity triangle. The CP violating phase $\gamma$ can be determined
cleanly as these decay modes are free from the penguin pollutions.
