Resum: Using finite energy sum rules, the hadronic matrix element of the DeltaB=2 operator governing the mixing between the B 0 and overlineB0 mesons is evaluated. The result is very sensitive to the input value of the bottom quark mass. Allowing it to vary in the range mb=4.70±0.14 GeV, one finds xi B|f B| |B B| =0.13±0.05 GeV.
