  The production cross sections of top-quark pairs in association with massive
vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} =
8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated
luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012
at the LHC. Final states with two, three or four leptons are considered. A fit
to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes
simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the
background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The
measured cross sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and
$\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with
neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All
measurements are consistent with next-to-leading-order calculations for the
$t\bar{t}W$ and $t\bar{t}Z$ processes.
