  HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the
X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We
use recent data from the NICER instrument, joined with archival data from
XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30
years. Contrary to previous works, that relied on parameters determined through
optical observations, the new timing solution could be derived using only X-ray
data. We confirm that the compact object is steadily spinning up with Pdot =
-2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major
axis of the compact object aX sini = 9.60(5) lightsec. This allows us to
determine the inclination and masses of the system as i = 84.5(7) deg, MX =
1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year)
and orbital variations of the soft X-ray pulsed flux, without finding evidence
for variability. In the light of the new findings, we discuss the nature of the
compact object, concluding that the possibility of a neutron star in the
subsonic propeller regime is unlikely, while accretion of the subdwarf wind
onto a massive white dwarf can explain the observed luminosity and spin-up rate
for a wind velocity of ~800 km/s.
