Effect of continuum placement uncertainties on the total CO column density, excitation temperature and Doppler parameter. The histograms represent the distributions of log N(CO), T_ex(CO) and b for 100 realisations where the continuum for each fitting region has been modified independently and randomly. The colour of each point indicates the reduced χ² that varies from 1.08 (darkest) to 1.23 (lightest). The blue (resp. red) error bar corresponds to the best fit value using the combined UVES spectrum (resp. both UVES spectra fitted simultaneously).