  We discuss a novel method to calculate fB on the lattice based on the study
of the dependence of finite size effects upon the heavy quark mass of flavoured
mesons and on a non--perturbative recursive finite size technique. This method
avoids the systematic errors related to extrapolations from the static limit or
to the tuning of the coefficients of effective Lagrangian and the results admit
an extrapolation to the continuum limit. We show the results of a first
estimate at finite lattice spacing, but close to the continuum limit, giving fB
= 170(11)(5)(22)MeV. We also obtain fBs = 192(9)(5)(24)MeV. The first error is
statistical, the second is our estimate of the systematic error from the method
and the third the systematic error from the specific approximations adopted in
this first exploratory calculation. The method can be generalized to two--scale
problems in lattice QCD.
