e.g., @bodewits2014; @darius2019]. The slow-rotator model assumes every facet of the nucleus is in equilibrium with the solar radiation incident upon it, with the rotational pole pointed at the Sun. As mentioned previously, the size of 21P’s nucleus, necessary to convert the active area to the active fraction of the whole surface, is not well constrained so far, with a radius ranging from 1 to 2 km. Hence, to estimate the active fraction of the surface we assumed a radius of 1.5$\pm$0.5 km. Moreover, we assumed a bond albedo of 5$\%$ and a 100$\%$ infrared emissivity [see e.g., @hearn1989; @mckay2018; @mckay2019]. We found that the active area of 21P during our monitoring campaign varied from $\sim$5 km$^{2}$ at 1.49 au pre-perihelion, reached a maximum of $\sim$12 km$^{2}$ at 1.07 au pre-perihelion, and decreased to $\sim$1 km$^{2}$ at 1.31 au post-perihelion. Table \[tab:subli\] shows the minimum and maximum active areas and active fraction for 21P using the slow-rotator model at some interesting heliocentric distances. We obtained different values in comparison with previous estimations given by @combi2019. The reason is twofold; first, the already mentioned discrepancy in the water production rates found via different observational techniques (see Section 3.2), and secondly, the 