  The cross-section to produce a $W$ boson in association with a top quark is
measured differentially with respect to several particle-level final-state
observable quantities. The measurements are performed using
$36.1$~\mbox{fb$^{-1}$} of $pp$ collision data at $\sqrt{s}=13~\mathrm{TeV}$
collected in 2015 and 2016, by the ATLAS detector at the LHC. Cross-sections
are measured in a fiducial phase-space defined by the presence of two charged
leptons and exactly one jet identified as containing $B$ hadrons. Measurements
are normalised to the fiducial cross-section, causing several of the main
uncertainties to cancel. The results are found to be in good agreement with
predictions from several Monte Carlo generators.
