Exclusive decays and masses of theBmesons

We report on the observation of exclusive decays of B mesons to modes containing a D or $D^{\mathrm{*}+}$ and one or two charged pions. From kinematic reconstruction we determine the masses of the neutral and charged B mesons to be 5280.6±0.8±2.0 and 5278.6±0.8±2.0 MeV, respectively. The mass difference is 2.0±1.1±0.3 MeV. Branching ratios for these modes are given. From the charged-particle momentum spectrum in Υ(4S) decay, we find the average branching fraction for B-meson decay into a D or $D^{\mathrm{*}}$ plus a charged pion to be (0.81±0.16)%. We find no signal for exclusive decays which would arise from the b→u transition and set upper limits for several such modes.