  Finite energy QCD sum rules with Legendre polynomial integration kernels are
used to determine the heavy meson decay constant $f_{B_c}$, and revisit $f_B$
and $f_{B_s}$. Results exhibit excellent stability in a wide range of values of
the integration radius in the complex squared energy plane, and of the order of
the Legendre polynomial. Results are $f_{B_c} = 528 \pm 19$ MeV, $f_B = 186 \pm
14$ MeV, and $f_{B_s} = 222 \pm 12$ MeV.
