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tokens: in gas phase is still debated. In dark cores, only about 0.05% is locked in the S-bearing molecules detected at millimeter wavelengths (see the fractional abundances derived for S-bearing species towards the dark cloud TMC1-CP in Table 5 ). Unfortunately, most of the plausible major sulphur reservoirs such as S, HS, and S 2 are either not observable or very difficult to observe at millimeter wavelengths and one needs to trust to chemical models to estimate the exact amount of sulphur in the gas phase. It is widely accepted that most of the sulphur is locked in icy grain mantles. This is corroborated by the observations of S-bearing species in comets. In a recent compilation, Bockelée-Morvan Biver (2017) estimated that the S/O elemental ratio in the comet ice is about 1.5%, which is consistent within a factor of approximately four with the solar value, H 2 S being the main sulphur reservoir. Hot cores are compact warm regions where the icy mantles evaporate releasing the sulphur budget to the gas. Orion KL is the best-studied hot core, with full spectral surveys at millimeter and submillimeter wavelengths. In Table 5 , we show the abundances of sulphur species that have been derived in this prototypical hot core. Gas phase H 2 S is the main sulphur reservoir in this hot core and the elemental gas-phase sulphur abundance is ~ 1.6 × 10 −6 ( Crockett et al. 2014 ). Fuente et al. (2019) obtained a similar value for the sulphur gas phase abundance of ~10 −6 in the edges of the TMC1 molecular cloud. Table 5 Comparison of fractional abundances with observational templates. References. TMC1: Agúndez Wakelam (2013) ; Cernicharo et al. (2018) and references therein. Orion-KL: Tercero et al. (2010) ; Esplugues et al. (2014) ; Crockett et al. (2014) . Comets: Calmonte et al. (2016) ; Bockelée-Morvan Biver (2017) . Fig. 5 Abundances of NS + as a functionof N (H 2 ). Black pointscorresponds to the detections reported by Cernicharo et al. (2018) . Red points corresponds to the core and PDR positions in the Horsehead. The black solid line depicts a linear fit to the data. Based on the WHISPER survey, we have determined a significant inventory of sulphur species and their abundances in the Horsehead PDR. Only one H 2 S line has been observed and the estimated value of the H 2 S abundance (~0.3−5 × 10 −9 ) is hence uncertain. It seems clear, however, that the abundance of gaseous H 2 S is far from that observed in Orion-KL. One possibility is that a large fraction of H 2 S remains in the solid state. This is expected since the dust temperature in this PDR is around ~20–30 K in the PDR ( Goicoechea et al. 2009 ), which is below the sublimation temperature of H 2 S, ~80 K ( Jiménez-Escobar Muñoz Caro 2011 ). The detections of SO + and NS + suggest that a significant fraction of sulphur is in the form of S + in the outer layers of this cloud. Sulphur chemistry is still poorly known, and especially surface chemistry reactions need further understanding. Oba et al.