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The chemical composition of extremely metal-poor stars (EMP stars; [Fe/H]<~-3) is a unique tracer of early nucleosynthesis in the Galaxy. As such starsare rare, we wish to find classes of luminous stars which can be studied athigh resolution. We aim to determine the detailed chemical composition of thetwo EMP stars CS30317-056 and CS22881-039, originally thought to be redhorizontal-branch (RHB) stars, and compare it to earlier results for EMP starsas well as to nucleosynthesis yields from various supernova (SN) models. In theanalysis, we discovered that our targets are in fact the two most metal-poor RRLyrae stars known. Our detailed abundance analysis, taking into account thevariability of the stars, is based on VLT/UVES spectra (R~ 43000) and 1D LTEOSMARCS model atmospheres and synthetic spectra. For comparison with SN modelswe also estimate NLTE corrections for a number of elements. We derive LTEabundances for the 16 elements O, Na, Mg, Al, Si, S, Ca, Sc, Ti, Cr, Mn, Fe,Co, Ni, Sr and Ba, in good agreement with earlier values for EMP dwarf, giantand RHB stars. Li and C are not detected in either star. NLTE abundancecorrections are newly calculated for O and Mg and taken from the literature forother elements. The resulting abundance pattern is best matched by model yieldsfor supernova explosions with high energy and/or significant asphericityeffects. Our results indicate that, except for Li and C, the surfacecomposition of EMP RR Lyr stars is not significantly affected by mass loss,mixing or diffusion processes; hence, EMP RR Lyr stars should also be usefultracers of the chemical evolution of the early Galactic halo. The observedabundance ratios indicate that these stars were born from an ISM polluted byenergetic, massive (25 - 40M*) and/or aspherical supernovae, but the NLTEcorrections for Sc and certain other elements do play a role in the choice ofmodel.

First stars XIII. Two extremely metal-poor RR Lyrae stars?