Left : Comparison between the opacity correction factor for the R_FUV/NUV ratio and the opacity at the fiducial wavelength. The dashed and dash-dotted lines represents the correlations of $e^{({\tau }_{\mathrm{F}\mathrm{U}\mathrm{V}}-{\tau }_{\mathrm{N}\mathrm{U}\mathrm{V}})}\simeq 2{\tau }_{550}$ for O-rich AGB stars and $e^{({\tau }_{\mathrm{F}\mathrm{U}\mathrm{V}}-{\tau }_{\mathrm{N}\mathrm{U}\mathrm{V}})}\simeq 0.025{\tau }_{550}$ for C-rich AGB stars respectively. Right : Comparison between R_FUV/NUV and the opacity at the fiducial wavelength. The colour and shape of the markers represent the stellar variability and chemistry type as in Fig. 4. Grey markers are those without correction from dust attenuation. The dotted lines link the markers for the same sources before and after the dust attenuation correction.