The H IX galaxy survey - III. The gas-phase metallicity in H I eXtreme galaxiesK. A. Lutz, V. Kilborn, B. Catinella, L. Cortese, T. H. Brown and B. KoribalskiA&A, 635 (2020) A69DOI: https://doi.org/10.1051/0004-6361/201936556