The fragmentation of b quarks into B mesons is studied with four million ha
dronic Z decays collected by the ALEPH experiment during the years 1991-199
5. A semi-exclusive reconstruction of B --> lvD((*)) decays is performed, b
y combining lepton candidates with fully reconstructed D-(*) mesons while t
he neutrino energy is estimated from the missing energy of the event. The m
ean value of x(B)(wd), the energy of the weakly-decaying B meson normalised
to the beam energy, is found to be (x(B)(wd)) = 0.716 +/- 0.006 (stat) +/-
0.006 (syst), using a model-independent method; the corresponding value fo
r the energy of the leading B meson is (x(B)(L)) = 0.736 +/- 0.006 (stat) /- 0.006 (syst). The reconstructed spectra are compared with different frag
mentation models. (C) 2001 Published by Elsevier Science B.V.