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We report the discovery and high-resolution, high S/N, spectroscopic analysisof the ultra-metal-poor red giant HE 0557-4840, which is the third mostheavy-element deficient star currently known. Its atmospheric parameters areT_eff = 4900 K, log g = 2.2, and [Fe/H]= -4.75. This brings the number of starswith [Fe/H] < -4.0 to three, and the discovery of HE 0557-4840 suggests thatthe metallicity distribution function of the Galactic halo does not have a"gap" between [Fe/H] = -4.0, where several stars are known, and the two mostmetal-poor stars, at [Fe/H] ~ -5.3. HE 0557-4840 is carbon rich - [C/Fe] = +1.6- a property shared by all three objects with [Fe/H] < -4.0, suggesting thatthe well-known increase of carbon relative to iron with decreasing [Fe/H]reaches its logical conclusion - ubiquitous carbon richness - at lowestabundance. We also present abundances (nine) and limits (nine) for a further 18elements. For species having well-measured abundances or strong upper limits,HE 0557-4840 is "normal" in comparison with the bulk of the stellar populationat [Fe/H] ~ -4.0 - with the possible exception of Co. We discuss theimplications of these results for chemical enrichment at the earliest times, inthe context of single ("mixing and fallback") and two-component enrichmentmodels. While neither offers a clear solution, the latter appears closer to themark. Further data are required to determine the oxygen abundance and improvethat of Co, and hence more strongly constrain the origin of this object.

HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich