084127

HD49798 is the only known X-ray binary with a hot subdwarf companion. The compact object openPara white dwarf or a neutron starclosePar has a spin period of 13 s and has been steadily spinning up at 2x10elevkeyOpen -15keyClose s-s for the last 20 years. Independently of its NS or WD nature comma this spin-up is difficult to explain with the angular momentum that accretion can supply. An alternative explanation implies the presence of a young WD comma still in the contracting phase. A new observation must be secured as soon as possible in order to extend the phase connected timing solution of this unique binary and shed light on thenature of the compact object. This fulfil proposal was accepted in AO17 openParC priorityclosePar but not executed.