  Context: Four open clusters are present in the Kepler field of view and
timeseries of nearly a year in length are now available. These timeseries allow
us to derive asteroseismic global oscillation parameters of red-giant stars in
the three open clusters NGC 6791, NGC 6819 and NGC 6811. From these parameters
and effective temperatures, we derive mass, radii and luminosities for the
clusters as well as field red giants. Aims: We study the influence of evolution
and metallicity on the observed red-giant populations. Methods: The global
oscillation parameters are derived using different published methods and the
effective temperatures are derived from 2MASS colours. The observational
results are compared with BaSTI evolution models. Results: We find that the
mass has significant influence on the asteroseismic quantities delta_nu vs.
nu_max relation, while the influence of metallicity is negligible, under the
assumption that the metallicity does not affect the excitation / damping of the
oscillations. The positions of the stars in the H-R diagram depend on both mass
and metallicity. Furthermore, the stellar masses derived for the field stars
are bracketed by those of the cluster stars. Conclusions: Both the mass and
metallicity contribute to the observed difference in locations in the H-R
diagram of the old metal-rich cluster NGC 6791 and the middle-aged
solar-metallicity cluster NGC 6819. For the young cluster NGC 6811, the
explanation of the position of the stars in the H-R diagram challenges the
assumption of solar metallicity, and this open cluster might have significantly
lower metallicity [Fe/H] in the range -0.3 to -0.7 dex. Also, nearly all the
observed field stars seem to be older than NGC 6811 and younger than NGC 6791.
