MEREGHETTI, SandroSandroMEREGHETTIPintore, FabioFabioPintoreESPOSITO, PAOLOPAOLOESPOSITOLA PALOMBARA, NICOLANICOLALA PALOMBARATIENGO, ANDREAANDREATIENGOISRAEL, Gian LucaGian LucaISRAELSTELLA, LuigiLuigiSTELLA2020-05-262020-05-2620160035-8711http://hdl.handle.net/20.500.12386/25190The hot subdwarf HD 49798 has an X-ray emitting compact companion with a spin-period of 13.2 s and a dynamically measured mass of 1.28 ± 0.05 M<SUB>☉</SUB>, consistent with either a neutron star or a white dwarf. Using all the available XMM-Newton and Swift observations of this source, we could perform a phase-connected timing analysis extending back to the ROSAT data obtained in 1992. We found that the pulsar is spinning up at a rate of (2.15 ± 0.05) × 10<SUP>-15</SUP> s s<SUP>-1</SUP>. This result is best interpreted in terms of a neutron star accreting from the wind of its subdwarf companion, although the remarkably steady period derivative over more than 20 yr is unusual in wind-accreting neutron stars. The possibility that the compact object is a massive white dwarf accreting through a disc cannot be excluded, but it requires a larger distance and/or properties of the stellar wind of HD 49798 different from those derived from the modelling of its optical/UV spectra.STAMPAenArticle10.1093/mnras/stw5362-s2.0-84965031061000375799500011https://academic.oup.com/mnras/article/458/4/3523/26138252016MNRAS.458.3523MFIS/05 - ASTRONOMIA E ASTROFISICA