  HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the mass
donor is a hot subdwarf star of O spectral type and, most likely, it contains a
massive white dwarf (1.28$\pm$0.05 M$_{\rm SUN}$) with a very fast spin period
of 13.2 s. Here we report the results of new XMM-Newton pointings of this
peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of
all the previous observations. The new data indicate that the compact object is
still spinning-up at a steady rate of $(-2.17\pm0.01)\times10^{-15}$ s
s$^{-1}$, consistent with its interpretation in terms of a young contracting
white dwarf. Comparison of observations obtained at similar orbital phases, far
from the ecplise, shows evidence for long term variability of the hard ($>$0.5
keV) spectral component at a level of $\sim$(70$\pm$20)\%, suggesting the
presence of time-dependent inhomogeneities in the weak stellar wind of the
HD49798 subdwarf. To investigate better the soft spectral component that
dominates the X-ray flux from this system, we computed a theoretical model for
the thermal emission expected from an atmosphere with element abundances and
surface gravity appropriate for this massive white dwarf. This model gives a
best fit with effective temperature of T$_{\rm eff}$=2.25$\times$10$^5$ K and
an emitting area with radius of $\sim$1600 km, larger than that found with
blackbody fits. This model also predicts a contribution of the pulsed emission
from the white dwarf in the optical band significantly larger than previously
thought and possibly relevant for optical variability studies of this system.
