  We have observed HNC 1-0, CN 1-0 and 2-1 line emission in a sample of 13 IR
luminous (LIRGs, L_IR > 10E11 Lo) starburst and Seyfert galaxies. HNC 1-0 is
detected in 9, CN 1-0 is detected in 10 and CN 2-1 in 7 of the galaxies. We
also report the first detection of HC3N (10-9) emission in Arp220. The
excitation of HNC and CN emission requires densities n > 10E4 cm-3. We compare
their intensities to that of the usual high density tracer HCN. The
I(HCN)/I(HNC}) and I(HCN)/I(CN) 1-0 line intensity ratios vary significantly,
from 0.5 to >6, among the galaxies. This implies that the actual properties of
the dense gas is varying among galaxies who otherwise have similar I(CO)/I(HCN)
line intensity ratios. We suggest that the HNC emission is not a reliable
tracer of cold (10 K) gas at the center of LIRGs, as it often is in the disk of
the Milky Way. Instead, the HNC abundance may remain substantial, despite high
gas temperatures, because the emission is emerging from regions where the HCN
and HNC formation and destruction processes are dominated by ion-neutral
reactions which are not strongly dependent on kinetic temperature. We find five
galaxies (four AGNs and one starburst) where the I(HCN)/I(HNC) intensity ratio
is close to unity. In other AGNs, however, I(HCN)/I(HNC}) is >4. The CN
emission is on average a factor of two fainter than HCN, but the variation is
large and there seems to be a trend of reduced relative CN luminosity with
increasing IR luminosity. One galaxy, NGC3690, has a CN luminosity twice that
of HCN and its ISM is thus strongly affected by UV radiation. We discuss the
I(HCN)/I(HNC) and I(HCN)/I(CN) line ratios as indicators of starburst
evolution.
