Aims. One of the primary objectives of the TOPoS survey is to search for the most metal-poor stars. Our search has led to the discovery
of one of the most iron-poor objects known, SDSS J092912.32+023817.0. This object is a multiple system, in which two components
are clearly detected in the spectrum.
Methods. We have analysed 16 high-resolution spectra obtained using the UVES spectrograph at the ESO 8.2 m VLT telescope to
measure radial velocities and determine the chemical composition of the system.
Results. Cross correlation of the spectra with a synthetic template yields a double-peaked cross-correlation function (CCF) for eight
spectra, and in one case there is evidence for the presence of a third peak. Chemical analysis of the spectrum obtained by averaging
all the spectra for which the CCF showed a single peak found that the iron abundance is [Fe/H]=–4.97. The system is also carbon
enhanced with [C/Fe]=+3.91 (A(C) = 7.44). From the permitted oxygen triplet we determined an upper limit for oxygen of [O/Fe]<
+3.52 such that C/O > 1.3. We are also able to provide more stringent upper limits on the Sr and Ba abundances ([Sr/Fe] < +0.70,
and [Ba/Fe] < +1.46, respectively).