Metal–poor turnoff and subgiant field stars in the Galaxy - I. RGU photometry in a high–latitude field near M101S. Güngör Ak, S. Karaali and R. BuserAstron. Astrophys. Suppl. Ser., 131 2 (1998) 345-354DOI: https://doi.org/10.1051/aas:1998429