Marino, A. F., Milone, A. P., Sneden, C., Bergemann, M., Kraft, R. P., Wallerstein, G., et al. (2012). The double sub-giant branch of NGC 6656 (M 22): a chemical characterization. Astronomy & Astrophysics, 541: A15, pp. 1-18. doi:10.1051/0004-6361/201118381.