  We explore the properties of 24 field early-type galaxies at 0.20<z<0.75 down
to M_B<=-19.30 in a sample extracted from the FORS Deep Field and the William
Herschel Deep Field. High S/N intermediate-resolution VLT spectroscopy was
complemented by deep high-resolution HST/ACS imaging and additional
ground-based multi-band photometry. To clarify the low level of star formation
(SF) detected in some galaxies, we identify the amount of AGN activity in our
sample using archive data of Chandra and XMM-Newton X-ray surveys. The B and
K-band Faber-Jackson relations and the Fundamental Plane display a moderate
evolution for the field early-type galaxies. Lenticular (S0) galaxies feature
on average a stronger luminosity evolution and bluer rest-frame colours which
can be explained that they comprise more diverse stellar populations compared
to elliptical galaxies. The evolution of the FP can be interpreted as an
average change in the dynamical mass-to-light ratio of our galaxies as <\Delta
\log{(M/L_B)}/z>=-0.74\pm0.08. The M/L evolution of these field galaxies
suggests a continuous mass assembly of field early-type galaxies during the
last 5 Gyr, that gets support by recent studies of field galaxies up to z~1.
Independent evidence for recent SF activity is provided by spectroscopic (OII
em., Hdelta) and photometric (rest-frame colors) diagnostics. Based on the
Hdelta absorption feature we detect a weak residual SF for galaxies that
accounts for 5%-10% in the total stellar mass of these galaxies. The
co-evolution in the luminosity and mass of our galaxies favours a downsizing
formation process. We find some evidence that our galaxies experienced a period
of SF quenching, possible triggered by AGN activity that is in good agreement
with recent results on both observational and theoretical side. (abridged)
