Comparison between CO spectra of K2 (observed in July with ESO/CRIRES⁺) and C/2006 W3 (Christensen) (observed in October 2009 with ESO/CRIRES). The comets were at heliocentric distances of 2.75 au and 3.25 au, respectively. In both panels, the red lines indicate the CO model, while the dotted yellow lines are the ±1σ noise. C/2006 W3 (Christensen) shows very strong CO emission lines even if at a larger heliocentric distance than K2 (bottom panel, adapted from Bonev et al. 2017), while CO lines for K2 are at the noise level (top panel, Lippi et al., in prep.).