  Using a data sample of pp collisions corresponding to an integrated
luminosity of $3~ \rm fb^{-1}$, the $\Xi_b^-$ and $\Omega_b^-$ baryons are
reconstructed in the $\Xi_b^- \rightarrow J/\psi \Xi^-$ and $\Omega_b^-
\rightarrow J/\psi \Omega^-$ decay modes and their lifetimes measured to be
  $\tau (\Xi_b^-) = 1.55\, ^{+0.10}_{-0.09}~{\rm(stat)} \pm 0.03\,{\rm(syst)}$
ps,
  $\tau (\Omega_b^-) = 1.54\, ^{+0.26}_{-0.21}~{\rm(stat)} \pm
0.05\,{\rm(syst)}$ ps.
  These are the most precise determinations to date. Both measurements are in
good agreement with previous experimental results and with theoretical
predictions.
