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

HD49798 is a unique binary composed of a 13 s pulsator, most likely the fastest-rotating massive white dwarf (1.28 Msun), orbiting the brightest O-type subdwarf. We recently obtained the first phase-connected timing solution, that should be extended over a longer period to unveil the small Pdot variations expected in this system. Their study will allow us to confirm the nature of the compact object, to shed light on the accretion flow geometry and on the magnetic field intensity, and to improve the accuracy on the orbital parameters of this interesting post-common envelope and possibly SN Ia progenitor system. Only if a new period measurement is obtained during the AO12 it will be possible to extend the phase-connection over a long time period with a small requirement on observing time.