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2S 0918-549 is a low-mass X-ray binary (LMXB) with a low optical to X-rayflux ratio. Probably it is an ultracompact binary with an orbital periodshorter than 60 min. Such binaries cannot harbor hydrogen rich donor stars. Aswith other (sometimes confirmed) ultracompact LMXBs, 2S 0918-549 is observed tohave a high neon-to-oxygen abundance ratio (Juett et al. 2001) which has beenused 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 implyingthe 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 Wefirst present a Type I X-ray burst from 2S 0918-549 with a long duration of 40minutes. We show that this burst is naturally explained by accretion of purehelium at the inferred accretion rate of ~0.01 times the Eddington accretionrate. At higher accretion rates of ~0.1 Eddington, hydrogen is required toexplain long duration bursts. However, at low rates the long duration is due tothe large amount of helium that accumulates prior to the burst. We show that itis possible to form a helium white dwarf donor in an ultracompact binary ifaccretion starts during the first ascent of the giant branch, when the core ismade of predominantly helium. Furthermore, this scenario naturally explains thehigh neon-to-oxgen ratio, without requiring a CO or ONe white dwarf companion.The only observational aspect of 2S 0918-549 that we cannot explain is theabsence of helium lines in the optical spectrum. Model calculations of opticalaccretion disk spectra need to be carried out in order to obtain limits on thehelium abundance.Comment: accepted by A&A, 11 pages, 10 figure

On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918–549