First observations of (B)over-bar(s)(0) -> D+D-, Ds+D- and D-0(D)over-bar(0) decays

First observations and measurements of the branching fractions of the Bbar0s→D+D−, Bbar0s→D+sD− and Bbar0s→D0Dbar0 decays are presented using 1.0  fb−1 of data collected by the LHCb experiment. These branching fractions are normalized to those of Bbar0→D+D−, B0→D−D+s and B−→D0D−s, respectively. An excess of events consistent with the decay Bbar0→D0Dbar0 is also seen, and its branching fraction is measured relative to that of B−→D0D−s. Improved measurements of the branching fractions ℬ(Bbar0s→D+sD−s) and ℬ(B−→D0D−s) are reported, each relative to ℬ(B0→D−D+s). The ratios of branching fractions are ℬ(Bbar0s→D+D−)/ℬ(Bbar0→D+D−)=1.08±0.20±0.10, ℬ(Bbar0s→D+sD−)/ℬ(B0→D−D+s)=0.050±0.008±0.004, ℬ(Bbar0s→D0Dbar0)/ℬ(B−→D0D−s)=0.019±0.003±0.003, ℬ(Bbar0→D0Dbar0)/ℬ(B−→D0D−s)<0.0024 at 90% CL, ℬ(Bbar0s→D+sD−s)/ℬ(B0→D−D+s)=0.56±0.03±0.04, ℬ(B−→D0D−s)/ℬ(B0→D−D+s)=1.22±0.02±0.07, where the uncertainties are statistical and systematic, respectively.