Context : Fast stellar rotation is currently the most promising mechanism for producing primary nitrogen in metal-poor massive stars .
Chemical evolution models computed with the inclusion of the yields of fast rotating models at a metallicity Z = 10 ^ { -8 } can account for the high N/O abundances observed in normal metal-poor halo stars .
If , as believed , intermediate mass stars did not have enough time to contribute to the interstellar medium enrichment at such low metallicities , the above result constitutes a strong case for the existence of fast rotators in the primordial Universe .

Aims : An important result of stellar models of fast rotators is that large quantities of primary ^ { 13 } \kern - 0.8 pt C are produced .
Hence , our goal is to investigate the consequence of fast rotation on the evolution of the ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C ratio in the interstellar medium at low metallicity .

Methods : We compute the evolution of the ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C ratio for the first time at very low metallicities upon the inclusion of fast rotators at Z = 10 ^ { -8 } .

Results : We predict that , if fast rotating massive stars were common phenomena in the early Universe , the primordial interstellar medium of galaxies with a star formation history similar to the one inferred for our galactic halo should have ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C ratios between 30-300 .
Without fast rotators , the predicted ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C ratios would be \sim 4500 at [ Fe/H ] = -3.5 , increasing to \sim 31000 at around [ Fe/H ] = -5.0 .
Current data on very metal-poor giant normal stars in the galactic halo agree better with chemical evolution models including fast rotators .
The expected difference in the ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C ratios , after accounting for the effects of the first dredge-up , between our predictions with/without fast rotators is of the order of a factor of 2-3 .
However , larger differences ( a factor of \sim 60 - 90 ) are expected for giants at [ Fe/H ] = -5 or turnoff stars already at [ Fe/H ] = -3.5 .
To test our predictions , challenging measurements of the ^ { 12 } \kern - 0.8 pt C / ^ { 13 } \kern - 0.8 pt C in more extremely metal-poor giants and turnoff stars are required .

Conclusions :