the BC\[4450 Å\] filter. After correcting their Af$\rho$ values for the phase angle effect using the same function as for the TRAPPIST data (see section \[Photometry\_TRAPPIST\]), both data sets are in agreement. This indicates that there is no evidence of changes in the chemical composition in the coma of the comet at different heliocentric distances (in the range 1.0 to 1.5 au) and over the five orbits, which is an argument to reject the evolutionary origin of the carbon chain depletion in that comet.

As seen in the bottom panel of Figure \[ratios\], there is also no evidence that the dust-to-gas ratio represented by A(0)f$\rho$/Q(CN) and A(0)f$\rho$/Q(OH) depends on the heliocentric distance. We found that this ratio in 21P is consistent with the average value of depleted comets and higher than the mean value of the typical comets as defined in [@AHearn1995] (see Table \[tab:abundance\]). [@Lara2003] obtained a value of Log\[A(0)f$\rho$/Q(CN)\]=-22.91$\pm$0.10 in 1998 which is in agreement with our measurement. Like for the gas relative abundances, we conclude that 21P coma does not show significant variation in the dust-to-gas ratio over the previous apparitions and it has a similar ratio as the depleted comets defined by [@AHearn1995].

The comparison with 