ROSAT/IUE discovery of a white dwarf companion to HD 33959C (F4V)

We report the discovery, from the ROSAT EUV and X-ray all-sky surveys, and from IUE spectra, of a white dwarf (WD) companion to HD 33959C ( = ADS 3824C, F4V, mv = 7.95). RE 0515 + 324 is among the 10 brightest EUV sources in the sky in the 100–200 Å band. It is also a strong soft X-ray source. IUE observations of candidate objects have revealed a previously uncatalogued hot DA WD, visually unresolved from HD 33959C, making the pair one of three or four suggested Sirius-like systems. The estimated EUV luminosity is of the order of 400 times greater than that of Sirius B, implying that the WD is much hotter ( cf. Teff = 24 000 K for Sirius B) and probably hydrogen-rich. This is confirmed by modelling of the WD atmosphere, which shows that Teff ≳ 46 000 K. We discuss these results in the context of WDs in general and Sirius-like systems in particular.