Toward a long-term phase-connected ephemeris for the unique X-ray binary HD49798

HD 49798 is a unique X-ray binary composed of a 13.2 s pulsator, most likely the fastest-rotating massive white dwarf (1.28+/-0.05 solar masses), orbiting the brightest subdwarf of O spectral type. We obtained a phase-connected timing solution, that should now be extended over a longer time period to unveil the small variations in the spin period expected in this system. Their study will allow us to unambiguously confirm the nature of the compact object, to shed light on the accretion flow geometry and on the magnetic field intensity. This is an updated version of a similar proposal accepted in C priority in AO12.