2 The sample

From the Queen Mary & Westfield College IRAS galaxy catalogue (QIGC, 17711 entries, Rowan-Robinson et al. 1991) we chose the 48 brightest sources with (1) $F_{\rm 60\mu m} > 3$ Jy and (2) $L_{\rm FIR} = L_{\rm 40-120~\mu m} > 10^{\rm 12}$ $L_{\odot}$ (based on $H_{\rm0} = 50$ kms^-1/Mpc). Throughout this paper, however, we use $H_{\rm0} = 75$ kms^-1/Mpc, thus, some of the sources reach only about 10 $^{\rm 11.5}$ $L_{\odot}$. In the course of the ISO mission 37 out of these 48 sources could be observed (77%), all objects having z < 0.2. Thus, the sample is representative for the nearby bright ULIRGs. In coordination with the other ISO Guaranteed Time programmes the two slightly less luminous objects NGC 6240 and Mrk 463 were included. Later we observed four more objects with higher redshifts ( 0.4 > z > 0.2, $F_{\rm 60~\mu m} \approx 1$ Jy) which were visible to the satellite.