Table 1: Beamsizes and efficiencies.
Transition $\nu$ [GHz] HPBW [''] $\eta_{\rm mb}$

OSO SEST OSO SEST

HCN 1-0 88.6 44 57 0.59 0.75

HNC 1-0 90.6 42 55 0.59 0.75

CN 1-0 113.5 34 46 0.50 0.70

CO 1-0 115.2 33 45 0.50 0.70

**Table 1:** Beamsizes and efficiencies.
Transition	$\nu$ [GHz]	HPBW ['']	$\eta_{\rm mb}$
		OSO	SEST	OSO	SEST
HCN 1-0	88.6	44	57	0.59	0.75
HNC 1-0	90.6	42	55	0.59	0.75
CN 1-0	113.5	34	46	0.50	0.70
CO 1-0	115.2	33	45	0.50	0.70

We have used the SEST and OSO 20 m telescope to measure the HNC 1-0 (90.663 GHz) and the CN 1-0 113.491 GHz (1-0, J=3/2-1/2, F=5/2-3/2) line intensity in a selection of 13 LIRGs and ULIRGs. We also include observations of NGC 1808 which is of lower luminosity. The selected galaxies all have global ${I({\rm CO}) \over I({\rm HCN})}$ 1-0 intensity ratios $$\mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil $\displaystyle ...$ 15 (apart from NGC 3256 and NGC 1808). For the southern galaxies observed with SEST, we were also able to measure the CN 2-1 line (226.874 GHz (2-1, J=5/2-3/2, F=7/2-5/2), 226.659 GHz (2-1 J=3/2-1/2 F=5/2-3/2)). The CN 1-0 113.191 GHz line (1-0 J=1/2-1/2 F=3/2-3/2) is shifted +806 ${\rm km\,s}^{-1}$ from the main line and we have obtained limits to its intensity in several cases. For Arp 220 the two CN 1-0 spingroups are blended because the line is very wide. Thus, even in the 1 GHz correlator backend (see below) it was necessary to observe CN 1-0 at two different LO settings and then join the spectra together to get enough baseline. For four galaxies the bandwidth was wide enough to also include the 90.983 GHz 10-9 line of HC₃N.

Observations were made in 1999 October (HNC, OSO), December (HNC, SEST) and 2000 June (CN, OSO), August (CN, SEST). For OSO, the system temperatures were typically 300 K for HNC and 500-600 K for CN. For SEST, typical system temperatures were 230K for the HNC measurements and 400K for both the 113GHz and the 226GHz CN observations. Pointing was checked regularly on SiO masers and the rms was found to be 2'' for OSO, and 3'' for SEST. Arp 220 was observed both with OSO (CN) and SEST (HNC). We have also measured the 115 GHz CO 1-0 and the HCN 1-0 lines for some galaxies where we did not have values from the literature. Beamsizes and efficiencies are shown in Table 1. For the OSO observations a 500 MHz filterbank was used for backends for all observations, and for some a 1 GHz autocorrelator was also used. For the SEST observations we alternated between a 500 MHz and 1 GHz backend depending on whether simultaneous observations with the 1 and 3 mm receiver were taking place. We used the software package xs (written by P. Bergman) to subtract baselines and add spectra.

2 Observations