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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^- ->
D^0 l^- anti-nu_l)/Gamma(B^- -> D X l^- anti-nu_l) = 0.210 +/- 0.017(stat.) +/- 0.021(syst.) , Gamma(B^- ->
D^{*0}l^- anti-nu_l)/Gamma(B^- -> D X l^- anti-nu_l) = 0.611 +/- 0.022(stat.) +/- 0.027(syst.) and Gamma(B^- ->
D^{**0}(D^(*)\pi)l^- anti-nu_l)/Gamma(B^- -> D X l^- anti-nu_l) = 0.173 +/- 0.017 (stat.) +/- 0.021~(syst.).
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