  We report on our first set of results for charm physics, using a mixed-action
setup with maximally twisted valence fermions on CLS $N_f=2+1$ ensembles. This
setup avoids the need of improvement coefficients to subtract $O(am_c)$
effects. The charm quark mass, $D$ and $D_s$ decay constants are computed on a
subset of CLS ensembles, which allows to take the continuum limit and
extrapolate to the physical pion mass, and assess the scaling properties.
Special attention is paid to the implementation of techniques to deal with
systematic uncertainties. Our results show excellent prospects for
high-precision computations on the full set of ensembles.
