  We reconsider two classical proposals for the determination of the angle
gamma of the unitarity triangle: B^\pm => chi_{c0} \pi^\pm => \pi^+\pi^-\pi^\pm
and B_s => rho^0 K_S => \pi^+ \pi^- K_S. We point out the relevance, in both
cases, of non resonant amplitudes, where the \pi^+ \pi^- pair is produced by
weak decay of a B^* (J^P=1^-) or B_0 (J^P=0^+) off-shell meson. In particular,
for the B decay channel, the inclusion of the B_0 pole completes some previous
analyses and confirms their conclusions, provided a suitable cut in the Dalitz
plot is performed; for the B_s decay the inclusion of the B^*, B_0 amplitudes
enhances the role of the tree diagrams as compared to penguin amplitudes, which
makes the theoretical uncertainty related to the B_s => rho^0 K_S decay process
less significant. While the first method is affected by theoretical
uncertainties, the second one is cleaner, but its usefulness will depend on the
available number of events to perform the analysis.
