  We determine the masses and pseudoscalar decay constants of D and D_s mesons
employing lattice QCD with non-perturbatively O(a) improved Wilson quarks and a
tree-level Symanzik-improved gauge action. Our analysis is based on the
large-volume N_f=2+1 ensembles using open boundary conditions, generated within
the CLS effort. The status of results presented here covers two lattice
spacings, a ~ 0.0854 fm and a ~ 0.0644 fm, and pion masses varied from 420 to
200 MeV. We also report on our implementation of distance preconditioning for
the calculation of heavy quark propagators and discuss the impact of the
resulting accuracy improvements on the extraction of charmed meson masses and
decay constants. This is part of a continuing analysis by the RQCD and ALPHA
Collaborations, aiming at a stable continuum extrapolation using several
lattice spacings. To extrapolate to the physical masses, we follow both, the
(2*m_l+m_s)=const. and the m_s=const. line in parameter space.
