  Context. The first generation of stars formed in the Galaxy left behind the
chemical signatures of their nucleosynthesis in the interstellar medium,
visible today in the atmospheres of low-mass stars formed afterwards. Sampling
the chemistry of low-mass provides insight into the first stars. Aims. We aim
to increase the samples of stars with extremely low metal abundances,
identifying ultra metal-poor stars from spectra with modest spectral resolution
and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable
metallicities and carbon abundances from such spectra. Methods. We carry-out
follow-up observations of faint, V>19, metal-poor candidates selected from SDSS
spectroscopy and observed with OSIRIS at GTC. The SDSS and follow-up OSIRIS
spectra have been analyzed using the FERRE code to derive effective
temperatures, surface gravities, metallicities and carbon abundances. In
addition, a well-known extremely metal-poor star has been included in our
sample to calibrate the analysis methodology. Results. We have observed and
analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars
in our sample have been confirmed as extremely metal-poor stars, in the
[Fe/H]<-3.3 regime. We report the recognition of J173403+644632, a
carbon-enhanced ultra metal-poor dwarf star with [Fe/H]=-4.3 and [C/Fe]=+3.1.
