Context : The most metal-poor stars are the relics of the early chemical evolution of the Galaxy .
Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars .
The aim is to observe a sample of extremely metal-poor star ( EMP stars ) candidates selected from the Sloan Digital Sky Survey Data Release 12 ( SDSS DR12 ) and determine their chemical composition .

Aims : We obtain medium resolution spectra of a sample of six stars using the X-Shooter spectrograph at the Very Large Telescope ( VLT ) and we used ATLAS models to compute the abundances .

Methods : Five stars of the sample have a metallicity [ Fe/H ] between -2.5 dex and -3.2 dex .
We confirm the recent discovery of SDSS J002314.00+030758.0 .
As a star with an extremely low [ Fe/H ] ratio .
Assuming the \alpha -enhancement [ Ca/Fe ] = +0.4 dex , we obtain [ Fe/H ] = -6.1 dex .

Results : We could also determine its magnesium abundance and found that this star exhibits a very high ratio [ Mg/Fe ] \geq +3.60 dex assuming [ Fe/H ] = -6.13 dex .
We determined the carbon abundance and found A ( C ) = 6.4 dex .
From this carbon abundance , this stars belongs to the lower band of the A ( C ) - [ Fe/H ] diagram .

Conclusions :