  A measurement of the charge asymmetry in top-quark pair ($t\bar{t}$)
production in association with a photon is presented. The measurement is
performed in the single-lepton $t\bar{t}$ decay channel using proton-proton
collision data collected with the ATLAS detector at the Large Hadron Collider
at CERN at a centre-of-mass-energy of 13 TeV during the years 2015-2018,
corresponding to an integrated luminosity of 139 fb$^{-1}$. The charge
asymmetry is obtained from the distribution of the difference of the absolute
rapidities of the top quark and antiquark using a profile likelihood unfolding
approach. It is measured to be $A_\text{C}=-0.003 \pm 0.029$ in agreement with
the Standard Model expectation.
