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From millimeter and optical observations of the Jupiter-family comet17P/Holmes performed soon after its huge outburst of October 24, 2007, wederive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN,establishing that HCN has the same non-terrestrial isotopic composition as CN.The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp)after a reanalysis of previously published measurements. These results arecompatible with HCN being the prime parent of CN in cometary atmospheres. The15N excess relative to the Earth atmospheric value indicates that N-bearingvolatiles in the solar nebula underwent important N isotopic fractionation atsome stage of Solar System formation. HCN molecules never isotopicallyequilibrated with the main nitrogen reservoir in the solar nebula before beingincorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCNand CN are measured to be consistent with the terrestrial value.

Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes