The chemical abundances in the most metal poor stars provide information of the nature of the first stars. Among the useful elements there is the 12C/13C isotopic ratio which , however, is difficult to measure and in particular at low metallicity. We show that the CEMP-no stars, Carbon enhanced with no neutron-capture elements, offer a unique opportunity to measure the carbon isotopic ratio at the lowest metallicities. We report on the results of the GTO ESPRESSO program on extremely poor stars where we have studied at very high resolution (R 140000) six of the most extreme metal poor stars known all with Fe/H] < -4.5 and dawn to -7. In all the program CEMP-no stars we succeed in measuring the ratio with values in the range 39<12C/13C<100, i.e. lower or comparable than the Solar value. A search of the literature returned other dwarfs or subgiant CEMP-no stars with [Fe/H] -4 showing similar values which were completely overlooked because they were unexpected. These stars have not experienced the first dredge up and 13C cannot be produced internally. They are also single, or long term binaries, and mass transfer from an evolved AGB companion, as occurs in the CEMP-s, is ruled out. Therefore, 13C should have been present in the gas from which the stars formed, requiring a primary production of 13C at the dawn of chemical evolution. Thus, 13C is one of the most abundant species produced by the first stars. This in contrast with the predictions of Chemical Evolution Models where no C-13 is predicted initially.
Since these stars belong to the second or third generation the 13C should have been produced by the progenitors in a new process acting only at very low metallicities. Rotation and very low metallicities in massive stars could induce the mixing of helium and hydrogen burning layers bringing 12C into the hydrogen burning layer and leading to considerable 13C production. However, as this happens is far to be understood.
