  We present our first set of results for charm physics, using the mixed-action
setup introduced in a companion paper. Maximally twisted Wilson valence
fermions are used on a sea of non-perturbatively $O(a)$-improved Wilson
fermions, made up by CLS $N_{\mathrm{\scriptstyle f}}=2+1$ ensembles. Our
charm-sector observables are free from $O(am_c)$ discretisation effects,
without need of tuning any improvement coefficient, and show continuum-limit
scaling properties consistent with leading cutoff effects of $O(a^2)$. We
consider a subset of CLS ensembles -- including four values of the lattice
spacing and pion masses down to 200 MeV -- allowing to take the continuum limit
and extrapolate to the physical pion mass. A number of techniques are
incorporated in the analysis in order to estimate the systematic uncertainties
of our results for the charm quark mass and the $D_{(s)}$-meson decay
constants. This first study of observables in the charm sector, where the
emphasis has been on the control of the methodology, demonstrates the potential
of our setup to achieve high-precision results.
