Identification of the 4486,4504A emission lines in O-type spectra

Inspired by an appeal to the community from Walborn (these proceedings) we decided to solve the long-standing problem concerning the nature of the 4486,4504A emission lines, which are frequently observed in O-type spectra and which are particularly prominent in supergiants. We claim that these lines emerge from sulfur, namely by de-excitation of a highly excited S IV doublet state. We prove this by an exploratory NLTE calculation with a detailed model atom.