  We compute the decay constants of the lowest ccbar-states with quantum
numbers J(PC)=0(-+) [eta_c], 1(--) [J/psi], and 1(+-) [hc] by using lattice QCD
and QCD sum rules. We consider the coupling of J/psi to both the vector and
tensor currents. Lattice QCD results are obtained from the unquenched (Nf=2)
simulations using twisted mass QCD at four lattice spacings, allowing us to
take the continuum limit. On the QCD sum rule side we use the moment sum rules.
  The results are then used to discuss the rate of eta_c --> gamma gamma decay,
and to comment on the factorization in B --> X K decays, with X being either
eta_c or J/psi.
