Measurement of the $\bar{B}^0$ and $B^{-}$ meson lifetimes

The lifetimes of the B0 and B- mesonsare measured using a sample of about four million hadronic Z decays collected from 1991 to 19 95 with the ALEPHdetector at LEP. The data sample has been recently reprocessed, achieving a substantial improvement in the tracking performance. Semileptonic decaysof B0 and B- mesonsare partially reconstructed by identifying events containing a leptonwith an associated D* or D0 meson. The proper timeof the B mesonis estimated from the measured decay length and the momentum of the D- leptonsystem. A fit to the proper timeof 1880 D* l- and 2856 D0 l- candidates yields the following results: tau B0 = 1.518 +- 0.053 +- 0.034 ps, tau B- = 1.648 +- 0.049 +- 0.035 ps, tau B- / tau B0 = 1.085 +- 0.059 +- 0.018.