Observation of doubly-charmed B decays at LEP

A search for doubly-charmed B decays with both charmed mesons reconstructed is performed, using about 3.8 million hadronic Z decays recorded with the ALEPH detector at LEP. A clear signal is observed in the channels ${\mathrm B\rightarrow D_s \bar D}(X)$ and ${\mathrm B\rightarrow D \bar D}(X)$ (where D can be either a D$^0$, a D$^+$ or a D$^{*+}$), providing the first direct evidence for doubly-charmed B decays involving no ${\mathrm D_s}$ production. Evidence for associated ${\mathrm K^0_S}$ and ${\mathrm K^{\pm}}$ production in the decays ${\mathrm B\rightarrow D \bar D}(X)$ is also presented and some candidates for completely reconstructed decays ${\mathrm B\rightarrow D_s \bar D}(n\pi)$, ${\mathrm B\rightarrow D \bar D K^0_S}$ and ${\mathrm B\rightarrow D \bar D K^\pm}$ are observed. Furthermore, candidates for the two-body Cabibbo suppressed decays ${\mathrm B^0\rightarrow D^{*-}D^{*+}}$ and ${\mathrm B^-\rightarrow D^{(*)0}D^{(*)-}}$ are also observed. Measurements of the corresponding branching fractions are extracted.