Gladstone, J., Copperwheat, C., Heinke, C., Roberts, T., Cartwright, T., Levan, A., & Goad, M. (2013). Optical Counterparts of the Nearest Ultraluminous X-Ray Sources. Astrophysical Journal Supplement, 206(2), Article 14. https://doi.org/10.1088/0067-0049/206/2/14