2S 0918–549 is a low-mass X-ray binary ( LMXB ) with a low optical to X-ray flux ratio .
Probably it is an ultracompact binary with an orbital period shorter than 60 min .
Such binaries can not harbor hydrogen rich donor stars .
As with other ( sometimes confirmed ) ultracompact LMXBs , 2S 0918–549 is observed to have a high neon-to-oxygen abundance ratio ( Juett et al .
2001 ) which has been used to argue that the companion star is a CO or ONe white dwarf .
However , type-I X-ray bursts have been observed from several of these systems implying the presence of hydrogen or helium on the neutron star surface .
In this paper , we argue that the companion star in 2S 0918–549 is a helium white dwarf We first present a Type I X-ray burst from 2S 0918–549 with a long duration of 40 minutes .
We show that this burst is naturally explained by accretion of pure helium at the inferred accretion rate of \sim 0.01 times the Eddington accretion rate .
At higher accretion rates of \sim 0.1 Eddington , hydrogen is required to explain long duration bursts .
However , at low rates the long duration is due to the large amount of helium that accumulates prior to the burst .
We show that it is possible to form a helium white dwarf donor in an ultracompact binary if accretion starts during the first ascent of the giant branch , when the core is made of predominantly helium .
Furthermore , this scenario naturally explains the high neon-to-oxgen ratio , without requiring a CO or ONe white dwarf companion .
The only observational aspect of 2S 0918–549 that we can not explain is the absence of helium lines in the optical spectrum .
Model calculations of optical accretion disk spectra need to be carried out in order to obtain limits on the helium abundance .