  The inclusive top quark pair ($t\bar{t}$) production cross-section
$\sigma_{t\bar{t}}$ has been measured in $pp$ collisions at $\sqrt{s}=7$ TeV
and $\sqrt{s}=8$ TeV with the ATLAS experiment at the LHC, using $t\bar{t}$
events with an opposite-charge $e\mu$ pair in the final state. The measurement
was performed with the 2011 7 TeV dataset corresponding to an integrated
luminosity of 4.6 fb$^{-1}$ and the 2012 8 TeV dataset of 20.3 fb$^{-1}$. The
cross-section was measured to be: $\sigma_{t\bar{t}}=182.9\pm 3.1\pm 4.2\pm 3.6
\pm 3.3$ pb ($\sqrt{s}=7$ TeV) and $\sigma_{t\bar{t}}=242.9\pm 1.7\pm 5.5\pm
5.1\pm 4.2$ pb ($\sqrt{s}=8$ TeV, updated as described in the Addendum), where
the four uncertainties arise from data statistics, experimental and theoretical
systematic effects, the knowledge of the integrated luminosity and of the LHC
beam energy. The results are consistent with recent theoretical QCD
calculations at next-to-next-to-leading order. Fiducial measurements
corresponding to the experimental acceptance of the leptons are also reported,
together with the ratio of cross-sections measured at the two centre-of-mass
energies. The inclusive cross-section results were used to determine the top
quark pole mass via the dependence of the theoretically-predicted cross-section
on $m_t^{\rm pole}$, giving a result of $m_t^{\rm pole}=172.9^{+2.5}_{-2.6}$
GeV. By looking for an excess of $t\bar{t}$ production with respect to the QCD
prediction, the results were also used to place limits on the pair-production
of supersymmetric top squarks $\tilde{t}_1$ with masses close to the top quark
mass decaying via $\tilde{t}_1\rightarrow t\tilde{\chi}^0_1$ to predominantly
right-handed top quarks and a light neutralino $\tilde{\chi}_0^1$, the lightest
supersymmetric particle. Top squarks with masses between the top quark mass and
177 GeV are excluded at the 95% confidence level.
