HD 49798/RX J0648.0–4418 is a peculiar X-ray binary with a hot subdwarf ( sdO ) mass donor .
The nature of the accreting compact object is not known , but its spin period P = 13.2 s and \dot { P } = -2.15 \times 10 ^ { -15 } s s ^ { -1 } , prove that it can be only either a white dwarf or a neutron star .
The spin-up has been very stable for more than 20 years .
We demonstrate that the continuous stable spin-up of the compact companion of HD 49798 can be best explained by contraction of a young white dwarf with an age \sim 2 Myrs .
This allows us to interpret all the basic parameters of the system in the framework of an accreting white dwarf .
We present examples of binary evolution which result in such systems .
If correct , this is the first direct evidence for a white dwarf contraction on early evolutionary stages .