Context : The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang .

Aims : We report the study of a primordial star situated in the centre of the constellation Leo ( SDSS J102915+172027 ) .

Methods : The star , selected from the low resolution-spectrum of the Sloan Digital Sky Survey , was observed at intermediate ( with X-Shooter at VLT ) and at high spectral resolution ( with UVES at VLT ) .
The stellar parameters were derived from the photometry .
The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections .

Results : An iron abundance of [ Fe/H ] =–4.89 makes SDSS J102915+172927 one of the lowest [ Fe/H ] stars known .
However , the absence of measurable C and N enhancements indicates that it has the lowest metallicity , { Z } \leq 7.40 \times 10 ^ { -7 } ( metal-mass fraction ) , ever detected .
No oxygen measurement was possible .

Conclusions : The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed .
Even lower metallicity stars may yet be discovered , with a chemical composition closer to the composition of the primordial gas and of the first supernovae .