For each GRB host (labelled in the top left corner of each panel), first panel: optical image (Sollerman et al. 2005; Mazzali et al. 2006; Thöne et al. 2008; Hjorth et al. 2012; Starling et al. 2011; Michałowski et al. 2018a) together with the green circles marking the positions of the pointings and the beam sizes of our CO(2-1) observations. GRB positions are marked with red circles. North is up and east is to the left. Other panels: corresponding CO(2-1) spectra. Vertical dotted lines show the velocity intervals within which the line fluxes were measured.