We present deep BVRI band images and H I line observations of the nearby, low surface brightness galaxy ESO 215-G?009, which were obtained with the Australian National University 2.3 m telescope and the Australia Telescope Compact Array, respectively. ESO 215-G?009 was selected from the HIPASS Bright Galaxy Catalog because it has the second highest H I mass-to-light ratio of the galaxies with measured B-band apparent magnitudes. We find that it is an isolated dwarf irregular galaxy with an old stellar population. We place an upper limit on the current star formation rate of ~ 2.5 × 10-3 ⊙ yr-1. The extended H I disk shows regular rotation (vrot = 51 ± 8 km s-1) and, at a column density of ~ 5.0 × 1019 atoms cm-2, can be traced out to over 6 times the Holmberg radius of the stellar component (radius at μB = 26.6 mag arcsec-2). After foreground star subtraction, we measure a B-band apparent magnitude of 16.13 ± 0.07 mag within a radius of 80''. The H I flux density is 122 ± 4 Jy km s-1 within a radius of 370''. Given a Galactic extinction of AB = 0.95 ± 0.15 mag, we derive an H I mass-to-light ratio of 22 ± 4 ⊙/L⊙,B for ESO 215-G?009. To our knowledge this is the highest HI/LB ratio for a galaxy to be confirmed by accurate measurement to date.

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