  The aim of the present paper is to quantify the dependence of the estimates
of luminosities and stellar mass of early-type galaxies on the different models
and model parameters which can be used to analyze the observational data. The
paper is organized in two parts. The first one analyzes the dependence of the
M/L ratios and of the k-corrections in different bands on model parameters
(IMF, metallicity, star formation history, age), assuming some among the most
popular spectrophotometric codes usually adopted to study the evolutionary
status of galaxies [...]. The second part of our work is dedicated to quantify
the reliability and systematics affecting the mass and luminosity estimates
obtained by means of the best fitting technique applied to the photometric SEDs
of early-type galaxies at $1<z<2$. To this end, we apply the best fitting
technique to some mock catalogs built on the basis of a wide set of models of
early-type galaxies. We then compare the luminosity and the stellar mass
estimated from the SED fitting with the true known input values. The goodness
of the mass estimate is found to be dependent on the mass estimator adopted to
derive it, but masses cannot anyhow be retrieved better than within a factor
2-3, depending on the quality of the available photometric data and/or on the
distance of the galaxies since more distant galaxies are fainter on average and
thus affected by larger photometric errors. Finally, we present a new empirical
mass estimator based on the K band apparent magnitude and on the observed (V-K)
colour. We show that the reliability of the stellar mass content derived with
this new estimator for early-type galaxies and its stability are even higher
than those achievable with the best classic estimators, with the not negligible
advantage that it does not need any multi-wavelength data fitting.
