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The abundances of chemical elements and their depletion factors are essentialparameters for understanding the composition of the gas and dust that areultimately incorporated into stars and planets. Sulfur is an abundant butpeculiar element in the sense that, despite being less volatile than otherelements (e.g., carbon), it is not a major constituent of dust grains indiffuse interstellar clouds. Here, we determine the gas-phase carbon-to-sulfurabundance ratio, [C]/[S], and the sulfur abundance [S] in a dense star-formingcloud from new radio recombination lines (RRLs) detected with the Yebes 40mtelescope - at relatively high frequencies (~40 GHz ~7 mm) and angularresolutions (down to 36'') - in the Orion Bar, a rim of the Orion MolecularCloud (OMC). We detect nine Cn\alpha RRLs (with n=51 to 59) as well as ninenarrow line features separated from the Cn\alpha lines by delta v=-8.4+/-0.3 kms^-1. Based on this velocity separation, we assign these features to sulfurRRLs, with little contribution of RRLs from the more condensable elements Mg,Si, or Fe. Sulfur RRLs lines trace the photodissociation region (PDR) of theOMC. In these predominantly neutral gas layers, up to A_V~4, the ions C+ and S+lock in most of the C and S gas-phase reservoir. We determine a relativeabundance of [C]_Ori/[S]_Ori=10.4+/-0.6 and, adopting the same [C]_Ori measuredin the translucent gas toward star theta^1 Ori B, an absolute abundance of[S]_Ori=(1.4+/-0.4)x10^-5. This value is consistent with emission models of theobserved sulfur RRLs if N(S+)~7x10^17 cm^-2 (beam-averaged). The [S]_Ori is the''initial'' sulfur abundance in the OMC, before an undetermined fraction of the[S]_Ori goes into molecules and ice mantles in the cloud interior. The inferredabundance [S]_Ori matches the solar abundance, thus implying that there islittle depletion of sulfur onto rocky dust grains, with D(S)=0.0+/-0.2 dex.

The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines