  A measurement of observables sensitive to effects of colour reconnection in
top-quark pair-production events is presented using 139 fb$^{-1}$ of 13$\,$TeV
proton-proton collision data collected by the ATLAS detector at the LHC. Events
are selected by requiring exactly one isolated electron and one isolated muon
with opposite charge and two or three jets, where exactly two jets are required
to be $b$-tagged. For the selected events, measurements are presented for the
charged-particle multiplicity, the scalar sum of the transverse momenta of the
charged particles, and the same scalar sum in bins of charged-particle
multiplicity. These observables are unfolded to the stable-particle level,
thereby correcting for migration effects due to finite detector resolution,
acceptance and efficiency effects. The particle-level measurements are compared
with different colour reconnection models in Monte Carlo generators. These
measurements disfavour some of the colour reconnection models and provide
inputs to future optimisation of the parameters in Monte Carlo generators.

High Energy Physics - Experiment