  Observations of 48 red-clump stars were obtained in the H band with the
PIONIER instrument installed at the Very Large Telescope Interferometer.
Limb-darkened angular diameters were measured by fitting radial intensity
profile I(r) to square visibility measurements. Half the angular diameters
determined have formal errors better than 1.2%, while the overall accuracy is
better than 2.7%. Average stellar atmospheric parameters (effective
temperatures, metallicities and surface gravities) were determined from new
spectroscopic observations and literature data and combined with precise Gaia
parallaxes to derive a set of fundamental stellar properties. These intrinsic
parameters were then fitted to existing isochrone models to infer masses and
ages of the stars. The added value from interferometry imposes a better and
independent constraint on the R-Teff plane. Our derived values are consistent
with previous works, although there is a strong scatter in age between various
models. This shows that atmospheric parameters, mainly metallicities and
surface gravities, still suffer from a non-accurate determination, limiting
constraints on input physics and parameters of stellar evolution models.
