  This article presents measurements of $t\bar{t}$ differential cross-sections
in a fiducial phase-space region, using an integrated luminosity of 3.2
fb$^{-1}$ of proton--proton data at a centre-of-mass energy of $\sqrt{s} = 13$
TeV recorded by the ATLAS experiment at the LHC in 2015. Differential
cross-sections are measured as a function of the transverse momentum and
absolute rapidity of the top quark, and of the transverse momentum, absolute
rapidity and invariant mass of the $t\bar{t}$ system. The $t\bar{t}$ events are
selected by requiring one electron and one muon of opposite electric charge,
and at least two jets, one of which must be tagged as containing a $b$-hadron.
The measured differential cross-sections are compared to predictions of
next-to-leading order generators matched to parton showers and the measurements
are found to be consistent with all models within the experimental
uncertainties with the exception of the POWHEG-Box + HERWIG++ predictions,
which differ significantly from the data in both the transverse momentum of the
top quark and the mass of the $t\bar{t}$ system.
