First stars: XIII. Two extremely metal-poor RR Lyrae stars - Hansen, C. J; Nordstrom, B; Bonifacio, P; Spite, M; Andersen, J; Beers, T C; Cayrel, R; Depagne, E; Molaro, P; Barbuy, Beatriz Leonor Silveira; Depagne, E; Francois, P; et al. (Astronomy & Astrophysics, 2011)