model used by the authors (fast-rotator), which is less appropriate to describe the cometary outgassing.

Dust properties {#sec_colors}
---------------

We computed the A(0)f$\rho$ parameter at 10000km, as defined by [@A'Hearn1984], using broad-band (Rc and Ic) and narrow-band dust continuum filters (RC,GC,BC) (see Table \[tab:rates\]). Figure \[fig:afrho\_BVRI\] shows its evolution as a function of time to perihelion. Our results are in very good agreement with those reported by [@Ehlert2019], as shown in the right bottom panel of Figure \[fig:gas\_rate\]. Like for the gas, the maximum was reached on August 17 with a value of (1646.1$\pm$12.8) cm in the red narrow-band RC filter. About the same value was reported on previous apparitions at the same heliocentric distance [@Schleicher1987; @Lara2003; @Pittichova2008]. For a detailed description of the dust environment and its evolution, a more sophisticated model should be used, such as the Monte Carlo model presented by [@moreno2012] which was used successfully in a number of cases [see e. g., @pozuelos2015; @moreno2016a; @moreno2016b; @moreno2017; @pozuelos2018]. Such a study will be presented in a separate paper.

We used the A(0)f$\rho$ values obtained with the narrow-band filters which are not contaminated by the gas emissions to derive the dust colours. The normalized reflectivity gradients between 