HNC/HCN ratios observed in comets as a function of heliocentric distance. Values derived in comets ISON and Lovejoy are indicated as red squares. We also show the values (empty circles) and upper limits (empty triangles) derived in the sample of 14 moderately active comets compiled by Lis et al. (2008). The empirical relation found by these authors is shown as a dashed line. Values derived at various heliocentric distances before perihelion in comet Hale-Bopp (Biver et al. 1997; Irvine et al. 1998) are also shown as blue crosses.