  The results of a search for direct pair production of top squarks in events
with two opposite-charge leptons (electrons or muons) are reported, using 36.1
fb$^{-1}$ of integrated luminosity from proton--proton collisions at
$\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider.
To cover a range of mass differences between the top squark $\tilde{t}$ and
lighter supersymmetric particles, four possible decay modes of the top squark
are targeted with dedicated selections: the decay $\tilde{t} \rightarrow b
\tilde{\chi}_{1}^{\pm}$ into a $b$-quark and the lightest chargino with
$\tilde{\chi}_{1}^{\pm} \rightarrow W \tilde{\chi}_{1}^{0}$, the decay
$\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ into an on-shell top quark and
the lightest neutralino, the three-body decay $\tilde{t} \rightarrow b W
\tilde{\chi}_{1}^{0}$ and the four-body decay $\tilde{t} \rightarrow b \ell \nu
\tilde{\chi}_{1}^{0}$. No significant excess of events is observed above the
Standard Model background for any selection, and limits on top squarks are set
as a function of the $\tilde{t}$ and $\tilde{\chi}_{1}^{0}$ masses. The results
exclude at 95% confidence level $\tilde{t}$ masses up to about 720 GeV,
extending the exclusion region of supersymmetric parameter space covered by
previous searches.
