  We present a study of B semileptonic decays into charm final states based on
211.7 fb^{-1} of data collected at the \Upsilon(4S) resonance with the babar
detector at the pep2 e^+e^- storage ring. Using a novel technique based on the
simultaneous fit of a set of variables reconstructed on the recoil of a B
tagged in an hadronic decay mode, we measure the relative branching fractions
\Gamma(B^- \to D^0\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^-
\bar{\nu}_{\ell}) = 0.210 \pm 0.017 (stat.) \pm 0.021 (syst.), \Gamma(B^- \to
D^{*0}\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^- \bar{\nu}_{\ell}) =
0.611 \pm 0.022 (stat.) \pm 0.027 (syst.) and \Gamma(B^- \to
D^{**0}(D^{(*)}\pi)\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^-
\bar{\nu}_{\ell}) = 0.173 \pm 0.017 (stat.) \pm 0.021 (syst.) .
