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The nature of the several microlensing events observed by the MACHO teamtowards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-masssubstellar objects and stars with masses larger than ~M_{sun} have been ruledout as major components of a Massive Astrophysical Compact Halo Object (MACHO)Galactic halo, while stars of half a solar mass seem to be viable candidates.Main sequence stars have been already discarded, and there are tightrestrictions on the role played by white dwarfs with hydrogen-dominatedatmospheres. In this paper we evaluate the contribution to the dark mattercontent of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. Forthis purpose we use a Monte Carlo simulator which incorporates up-to-dateevolutionary sequences of white dwarfs with hydrogen-rich andhydrogen-deficient atmospheres. We also take into account detailed descriptionsof the thick disk and the halo of our Galaxy as well as of a reliable model ofthe LMC. We find that the contribution of white dwarfs with hydrogen-deficientatmospheres moderately increases the theoretical estimate of the optical depthwith respect to the value obtained when only hydrogen-rich white dwarfs areconsidered. We also find that the contribuiton of the thick disk population ofwhite dwarfs is comparable to the halo contribution. However, the contributionsof both the halo and the thick disk white-dwarf populations are stillinsufficient to explain the number of events observed by the MACHO team.Finally, we find that the contribution to the halo dark matter of the entirepopulation under study is less than 10% at the 95% conficence level.

White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy