Comparison with known similar systems
To our knowledge, the only other ‘known’ system of this kind was the progenitor of the He nova V445 Pup. A pre-outburst luminosity of log(L/L⊙) = 4.34 ± 0.36 would be compatible with a 1.2–1.3 M⊙ star burning helium in a shell. No optical spectrum exists of the progenitor; the post-outburst spectra are H-deficient, with the strongest lines being C II and Fe II. On the basis of photographic plates taken before the outburst, an optical modulation by a factor of 1.25 and a period of 0.650654(10) days was found and interpreted as orbital variation of a common-envelope binary. There are three possibilities for the X-ray non-detection: (1) the flux oscillations during burning with phases of low luminosity, (2) the substantial Galactic foreground absorption in the case that the X-ray spectrum was as similarly soft as [HP99] 159 or (3) an only slightly lower temperature as compared with [HP99] 159, which would shift the emission below the X-ray detection window. Thus, the progenitor of the He nova V445 Pup could have been an object similar to [HP99] 159.
