  We investigate the most plausible stellar Initial Mass Function (IMF) and the
main origin of the tilt of the Fundamental Plane (FP) for old, massive
early-type galaxies. We consider a sample of 13 bright galaxies of the Coma
cluster and combine our results with those obtained from a sample of 57 lens
galaxies in the same luminous mass range. We estimate the luminous mass and
stellar mass-to-light ratio values of the sample galaxies by fitting their SDSS
multi-band photometry with composite stellar population models computed with
different dust-free, solar-metallicity templates and IMFs. We compare these
measurements and those derived from two-component orbit-based dynamical
modelling. The photometric and dynamical luminous mass estimates of the
galaxies in our sample are consistent, within the errors, if a Salpeter IMF is
adopted. On the contrary, with a Kroupa or Chabrier IMF the two luminous mass
diagnostics differ at a more than 4 \sigma level. For the massive Coma
galaxies, their stellar mass-to-light ratio scales with luminous mass as the
corresponding effective quantities are observed to scale on the FP. This
indicates that the tilt of the FP is primarily caused by stellar population
properties. We conclude that old, massive lens and non-lens early-type galaxies
obey the same luminous and dynamical scaling relations, favour a Salpeter IMF,
and suggest a stellar population origin for the tilt of the FP. The validity of
these results for samples of early-type galaxies with different age and mass
properties still remains to be tested.
