  The differential cross-section for the production of a $W$ boson in
association with a top quark is measured for several particle-level
observables. The measurements are performed using 36.1 fb$^{-1}$ of $pp$
collision data collected with the ATLAS detector at the LHC in 2015 and 2016.
Differential cross-sections are measured in a fiducial phase space defined by
the presence of two charged leptons and exactly one jet matched to a
$b$-hadron, and are normalised with the fiducial cross-section. Results are
found to be in good agreement with predictions from several Monte Carlo event
generators.
