Long-term phase-connected timing of the unique X-ray binary HD49798

HD49798/RX J0648.04418 is the only known Xray binary in which the donor is ahot subdwarf of O spectral type. The compact object (a white dwarf or a neutronstar) has a spin period of 13.2 s and has been steadily spinning up at2x10^{15} s/s for the last 20 years. Independently of its NS or WD nature, thisspinup is difficult to explain with the angular momentum that accretion cansupply. An alternative explanation requires the presence of a young WD, still incontracting phase. We propose a fulfil observation to extend the phase connectedtiming solution in order to investigate the timing properties of this uniquebinary and shed light on the nature of the compact object. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]