Measurement of the B-->D*lnu branching fractions and ||Vcb||

We study the exclusive semileptonic B meson decays B - →D* 0 l - ν ¯ and B ¯0 →D* + l - ν ¯ using data collected with the CLEO II detector at the Cornell Electron-positron Storage Ring (CESR). We present measurements of the branching fractions scrB(B ¯0 →D* + l - ν ¯ )= (0.5/f 00 )[4.49±0.32(stat.)±0.39 (syst.)]% and scrB(B - →D* 0 l - (ν ¯ )= (0.5/f +- )[5.13±0.54 (stat)±0.64 (syst)]%, where f 00 and f +- are the neutral and charged B meson production fractions at the Υ(4S) resonance, respectively. Assuming isospin invariance and taking the ratio of charged to neutral B meson lifetimes measured at higher energy machines, we determine the ratio f +- /f 00 =1.04±0.13 (stat) ±0.12 (syst) ±0.10 (lifetime); further assuming f +- +f 00 =1 we also determine the partial width Γ(B ¯ →D*lν ¯ )=[29.9±1.9 (stat)±2.7 (syst.)±2.0 (lifetime)] ns -1 (independent of f +- /f 00 ).