  The ratios of partial rates for charged B decays to the recently observed
$D^0 K$ mode and to the two $D_{CP}K$ final states (CP = $\pm$) are shown to
constrain the weak phase $\gamma \equiv {\rm Arg} (V_{ub}^*)$. The smaller
color-suppressed rate, providing further information about the phase, can be
determined from these rates alone. Present estimates suggest that, while the
first constraints can already be obtained in a high luminosity e^+e^- B
factory, measuring the color-suppressed rate would require dedicated hadronic B
production experiments.
