  We investigate the masses and decay constants of eta and eta' mesons using
the Wilson twisted mass formulation with N_f=2+1+1 dynamical quark flavours
based on gauge configurations of ETMC. We show how to efficiently subtract
excited state contributions to the relevant correlation functions and estimate
in particular the eta' mass with improved precision. After investigating the
strange quark mass dependence and the continuum and chiral extrapolations, we
present our results for masses and mixing angle(s) at the physical point. Using
chiral perturbation theory we also extract the decay constants f_l and f_s and
use them to estimate the decay widths of eta,\eta' to gamma gamma and the
transition form factor in the limit of large momentum transfer.
