The ^ { 12 } C ^ { 14 } N/ ^ { 12 } C ^ { 15 } NÂ and ^ { 12 } C ^ { 14 } N/ ^ { 13 } C ^ { 14 } NÂ isotopic ratios have been determined in comets C/1995 O1 ( Hale-Bopp ) , C/2001 Q4 ( NEAT ) and C/2003 K4 ( LINEAR ) at heliocentric distances of , respectively , 2.7 , 3.7 and 2.6 AU .
These ratios have also been measured at r \sim 1 AU .
No significant differences were found between all determinations , nor with the value obtained for other comets .
If confirmed , the discrepancy between the nitrogen isotopic ratios from optical and millimeter measurements on CN and HCN would rule out HCN as a major parent of the cometary CN radicals .