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Neutron star mergers are believed to be a major cosmological source of rapidneutron-capture elements. The kilonovae associated with neutron star mergershave to date yielded only a single well-identified spectral signature: the PCygni line of Sr$^+$ at about 1$\mu$m in the spectra of the optical transient,AT2017gfo. Such P Cygni lines are important, because they provide significantinformation not just potentially on the elemental composition of the mergerejecta, but also on the velocity, geometry, and abundance stratification of theexplosion. In this paper we show evidence for a previously unrecognised P Cygniline in the spectra of AT2017gfo that emerges several days after the explosion,located at $\lambda \approx 760\,$nm. We show that the feature iswell-reproduced by 4d$^2$-4d5p transitions of Y$^+$, which have a weighted meanwavelength around 760-770 nm, with the most prominent line at 788.19 nm. Whilethe observed line is weaker than the Sr$^+$ feature, the velocitystratification of the new line provides an independent constraint on theexpansion rate of the ejecta similar to the constraints from Sr$^+$.

Discovery of a 760 nm P Cygni line in AT2017gfo: Identification of yttrium in the kilonova photosphere