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 that formed afterwards .
Sampling the chemistry of those 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 the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy ( OSIRIS ) at GTC .
The SDSS and follow-up OSIRIS spectra were 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 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 \left [ { Fe / H } \right ] < -3.3 regime .
We report the recognition of J173403+644632 , a carbon-enhanced ultra metal-poor dwarf star with \left [ { Fe / H } \right ] = -4.3 and \left [ { C / Fe } \right ] = +3.1 .

Conclusions :