Observed spectra of CO isotopologues (in T_MB scale) towards L1489-IRS (left column) and B5-IRS1 (right column). Species, transition, and upper level energy are reported in each panel. For the panels in the left column the vertical dashed lines stand for the source systemic velocity as measured using C¹⁸O (2-1) emission (+7.3 km s⁻¹). For the panels in the right column the vertical dashed lines stand for the source systemic velocity measured using C¹⁸O (2-1) emission (+10.2 km s⁻¹). Top panels: CO (2-1) transition. Middle panels: Zoom-in on the CO (2-1) line wings (in black) and the ¹³CO (2-1) line wings (in magenta), scaled by a factor of 77 in the T_MB range from -2 K to 2.5 K. Bottom panels: ¹³CO (in green) and C¹⁸O (in red) scaled by a factor of 7.27 assuming isotopic ratios of ¹²C/¹³C = 77 and ¹⁶O/¹⁸O = 560 (Milam et al. 2005).