  We study heavy-heavy and heavy-light quark systems for charm with a
relativistic heavy quark action in 2+1 flavor lattice QCD. Configurations are
generated by the PACS-CS Collaboration at the lattice spacing is $a=0.09$ fm
with the lattice size of $32^3\times 64$ employing the $O(a)$-improved Wilson
quark action and the Iwasaki gauge action. We present preliminary results for
the charmonium spectrum and the $D$ and $D_s$ meson decay constants evaluated
at 3.5 MeV$< m_{\rm ud}<$ 12 MeV with $m_{\rm s}$ around the physical value. We
investigate the dynamical quark mass dependences of the hyperfine and the
orbital splittings. The decay constants are compared with the recent
experimental values.
