CO(1–0) line luminosities as a function of redshift for our sample. In some cases, error bars are smaller than the marker. The comparison samples are described in Fig. 5. Where necessary, all quantities have been corrected for magnification. On average, the unobscured QSOs targeted in this study have lower CO luminosities than other obscured QSOs and star–forming galaxies at similar redshift. It is thanks to gravitational lensing that we are able to detect these systems with short exposure times.