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Fig. 2.

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G/R intensity ratio as a function of the heliocentric distance (AU). The same symbol is used for the different spectra of each comet. All points except C/2017 K2 (Pan-STARRS) are taken from Decock et al. (2013). The vertical line represents the distance out to which the sublimation of water is strongly decreasing (Crovisier & Encrenaz 2000).

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