Context. The known mega metal-poor (MMP) and hyper metal-poor (HMP) stars, with [Fe/H] < −6.0 and < −5.0, respectively, likely belong to the CEMP-no class, namely, carbon-enhanced stars with little or no second peak neutron-capture elements. They are likely second-generation stars, and the few elements measurable in their atmospheres are used to infer the properties of a single or very few progenitors.

Aims. The high carbon abundance in the CEMP-no stars offers a unique opportunity to measure the carbon isotopic ratio, which directly indicates the presence of mixing between the He- and H-burning layers either within the star or in the progenitor(s). By means of high-resolution spectra acquired with the ESPRESSO spectrograph at the VLT, we aim to derive values for the ¹²C/¹³C ratio at the lowest metallicities.

Methods. We used a spectral synthesis technique based on the SYNTHE code and on ATLAS models within a Markov chain Monte Carlo methodology to derive ¹²C/¹³C in the stellar atmospheres of four of the most metal-poor stars known: the MMP giant SMSS J0313–6708 ([Fe/H] < −7.1), the HMP dwarf HE 1327–2326 ([Fe/H] = −5.8), the HMP giant SDSS J1313–0019 ([Fe/H] = −5.0), and the ultra metal-poor subgiant HE0233 -0343 ([Fe/H] = −4.7). We also revised a previous value for the MMP giant SMSS J1605–1443 ([Fe/H] = −6.2).

Results. In four stars we derive an isotopic value while for HE 1327–2326 we provide a lower limit. All measurements are in the range 39 < ¹²C/¹³C < 100, showing that the He- and H-burning layers underwent partial mixing either in the stars or, more likely, in their progenitors. This provides evidence of a primary production of ¹³C at the dawn of chemical evolution. CEMP-no dwarf stars with slightly higher metallicities show lower isotopic values, <30 and="" even="" approaching="" the="" cno="" cycle="" equilibrium="" value="" thus="" extant="" data="" suggest="" presence="" of="" a="" discontinuity="" in="" sup="">12C/¹³C ratio at around [Fe/H] ≈ − 4, which could mark a real difference between the progenitor pollution captured by stars with different metallicities. We also note that some MMP and HMP stars with high ¹²C/¹³C show low ⁷Li values, providing an indication that mixing in the CEMP-no progenitors is not responsible for the observed Li depletion.