  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 stars
are rare, we wish to find classes of luminous stars which can be studied at
high resolution. We aim to determine the detailed chemical composition of the
two EMP stars CS30317-056 and CS22881-039, originally thought to be red
horizontal-branch (RHB) stars, and compare it to earlier results for EMP stars
as well as to nucleosynthesis yields from various supernova (SN) models. In the
analysis, we discovered that our targets are in fact the two most metal-poor RR
Lyrae stars known. Our detailed abundance analysis, taking into account the
variability of the stars, is based on VLT/UVES spectra (R~ 43000) and 1D LTE
OSMARCS model atmospheres and synthetic spectra. For comparison with SN models
we also estimate NLTE corrections for a number of elements. We derive LTE
abundances 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, giant
and RHB stars. Li and C are not detected in either star. NLTE abundance
corrections are newly calculated for O and Mg and taken from the literature for
other elements. The resulting abundance pattern is best matched by model yields
for supernova explosions with high energy and/or significant asphericity
effects. Our results indicate that, except for Li and C, the surface
composition of EMP RR Lyr stars is not significantly affected by mass loss,
mixing or diffusion processes; hence, EMP RR Lyr stars should also be useful
tracers of the chemical evolution of the early Galactic halo. The observed
abundance ratios indicate that these stars were born from an ISM polluted by
energetic, massive (25 - 40M*) and/or aspherical supernovae, but the NLTE
corrections for Sc and certain other elements do play a role in the choice of
model.
