Exclusive decays and masses of the<i>B</i>mesons

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\ifmmode\pm\else\textpm\fi{}0.8\ifmmode\pm\else\textpm\fi{}2.0 and 5278.6\ifmmode\pm\else\textpm\fi{}0.8\ifmmode\pm\else\textpm\fi{}2.0 MeV, respectively. The mass difference is 2.0\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\pm\else\textpm\fi{}0.3 MeV. Branching ratios for these modes are given. From the charged-particle momentum spectrum in \ensuremath{\Upsilon}(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\ifmmode\pm\else\textpm\fi{}0.16)%. We find no signal for exclusive decays which would arise from the b\ensuremath{\rightarrow}u transition and set upper limits for several such modes.