Ultra metal-poor stars ([Fe/H]<-4.0) are very rare, and finding them is a challenging task. Both narrow-band photometry and low-resolution spectroscopy have been useful tools for identifying candidates, and in this work we combine both approaches. We cross-matched metallicity-sensitive photometry from the Pristine survey with the low-resolution spectroscopic LAMOST database, and re- analysed all LAMOST spectra with [Fe/H]Pristine<-2.5. We find that ~1/3rd of this sample (selected without [Fe/H]_Pristine quality cuts) also have spectroscopic [Fe/H]<-2.5. From this sample, containing many low signal-to- noise (S/N) spectra, we selected eleven stars potentially having [Fe/H]<-4.0 or [Fe/H]<-3.0 with very high carbon abundances, and we performed higher S/N medium-resolution spectroscopic follow-up with OSIRIS on the 10.4m Gran Telescopio Canarias (GTC). We confirm their extremely low metallicities, with a mean of [Fe/H]=-3.4 and the most metal-poor star having [Fe/H]=-3.8. Three of these are clearly carbon-enhanced metal-poor (CEMP) stars with +1.65<[C/Fe]<+2.45. The two most carbon-rich stars are either among the most metal-poor CEMP-s stars or the most carbon-rich CEMP-no stars known, the third is likely a CEMP-no star. We derived orbital properties for the OSIRIS sample and find that only one of our targets can be confidently associated with known substructures/accretion events, and that three out of four inner halo stars have prograde orbits. Large spectroscopic surveys may contain many hidden extremely and ultra metal-poor stars, and adding additional information from e.g. photometry as in this work can uncover them more efficiently and confidently.

Cone search capability for table J/MNRAS/519/5554/table1 (*EMP candidates from Pristine and LAMOST)

Cone search capability for table J/MNRAS/519/5554/table2 (Summary of OSIRIS observations)