Extracting γ from Bd(t) -> D(*)+ D(*)- and Bd -> Ds(*)+ D(*)- Decays

We show that the CP phase \gamma can be obtained from measurements of Bd(t) -> D(*)+ D(*)- and Bd -> Ds(*)+ D(*)-. These decays are related by flavor SU(3) in the limit where spectator-quark contributions are small. If the pseudoscalar-pseudoscalar decays Bd(t) -> D+ D- and Bd -> Ds+ D- are used, the leading-order SU(3)-breaking effect is f_Ds/f_D. The dependence on decay constants can be removed by using a double ratio involving two helicity states of the vector-vector decays Bd(t) -> D*+ D*- and Bd -> Ds*+ D*-. In this case the theoretical error arising from all sources is in the range 5-10%.