We started monitoring 21P with TRAPPIST-North (hereafter TN) at the beginning of June 2018 when the comet was at 1.55 au from the Sun. The comet was then observed from the southern hemisphere with TRAPPIST-South (hereafter TS) from the beginning of September 2018. The pair of TRAPPIST telescopes [@Jehin2011] is in such a case very useful as it allowed a continuous monitoring of the comet before and after perihelion. We collected images with the cometary HB narrow-band filters [@Farnham2000] to measure the production rates of the radicals OH, NH, CN, C$_3$, and C$_2$. We also acquired images with the dust continuum filters BC, GC, and RC for blue, green and red [@Farnham2000]. We used the broad-band filters B, V, Rc, and Ic [@Bessell1990] to compute the Af$\rho$ parameter which is a proxy of the dust production rate [@A'Hearn1984] and to derive the dust colours.

Throughout the passage of the comet, we made a high cadence monitoring of 21P with images taken about twice a week. On photometric nights, we also obtained long series of observations with the gas narrow-band filters, especially CN and C$_2$ filters, to measure the variations of the production rates during the same night due to the 