  Extremely metal poor (EMP) stars in the Milky Way inherited the chemical
composition of the gas out of which they formed. They therefore carry the
chemical fingerprint of the first stars in their spectral lines. It is commonly
assumed that EMP stars form from gas that was enriched by only one progenitor
supernova ('mono-enriched'). However, recent numerical simulations show that
the first stars form in small clusters. Consequently, we expect several
supernovae to contribute to the abundances of an EMP star ('multi-enriched').
We analyse seven recently observed EMP stars from the TOPoS survey by applying
the divergence of the chemical displacement and find that J1035+0641 is
mono-enriched ($p_{mono}=53\%$) and J1507+0051 is multi-enriched
($p_{mono}=4\%$). For the remaining five stars we can not make a distinct
prediction ($p_{mono} \lesssim 50\%$) due to theoretical and observational
uncertainties. Further observations in the near-UV will help to improve our
diagnostic and therefore contribute to constrain the nature of the first stars.
