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We present a Magellan/MIKE high-resolution (R ~ 35,000) spectrum of theancient star SD 1313-0019 which has an iron abundance of [Fe/H] = -5.0, pairedwith a carbon enhancement of [C/Fe] ~ 3.0. The star was initially identified byAllende Prieto et al. in the BOSS survey. Its medium-resolution spectrumsuggested a higher metallicity of [Fe/H] = -4.3 due to the CaII K line blendingwith a CH feature which is a common issue related to the search for the mostiron-poor stars. This star joins several other, similar stars with [Fe/H] <-5.0 that all display a combination of low iron and high carbon abundances.Other elemental abundances of SD 1313-0019 follow that of more metal-rich halostars. From fitting the abundance pattern with yields of Population IIIsupernova, we conclude that SD 1313-0019 had only one massive progenitor starwith 20 - 30 M_sun that must have undergone a mixing and fallback episode.Overall, there are now five stars known with [Fe/H] < -5.0 (1D LTE abundances).This population of second-generation stars strongly suggests massive firststars that almost exclusively produced large amounts of carbon through stellarwinds and/or their mixing and fallback supernova explosions. As a consequence,their natal clouds -- presumably some early minihalo structures -- containedample amounts of carbon and oxygen that likely facilitated the formation ofthese first low-mass stars.

SD 1313-0019 -- Another second-generation star with [Fe/H] = -5.0, observed with the Magellan Telescope