different methods, including comparison between HCN and CN production rates [@Rauer2003; @Opitom2015], coma morphologies [@Woodney2002], and also carbon and nitrogen isotopic ratios in both species [@Manfroid2009; @BockeleMorvan2015]. We should note however that in some cases both abundances do not agree and that other sources, for instance extended sources, have been claimed for the CN origin [@Fray2005].

Some molecules such as C$_4$H$_2$, CH$_2$C$_2$H$_2$, and CH$_3$C$_2$H are proposed to be the parent molecules of C$_3$ [@Helbert2005; @Mumma2011; @Holscher2015PhD], but these complex species were not observed at infrared or at radio wavelengths. NH and NH$_2$ were found to be depleted in 21P in the previous apparitions [@AHearn1995; @Fink2009]. New infrared observations in 2018 show very low NH$_3$ in 21P, with an upper limit ratio of Q(NH$_3$)/Q(H$_2$O) &lt; 0.6% [@Faggi2019]. In this work, we derive Q(NH)/Q(OH)=0.2% which is consistent with Q(NH$_3$)/Q(H$_2$O).

Optical high-resolution spectrum {#sec_ratios}
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Water production rate {#uves-water-production-rate}
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The UVES spectrum offered the possibility of computing independently the water production rate at the time of observation. We first measured the overall flux for the OH (0,0) band near 309 nm, integrated over the whole slit. We found 1.47$\times$10$^{-13}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. The fluorescence efficiency computed for this band and the 