Precision study of hyperfine structure in simple atoms
  We consider the most accurate tests of bound state QED theory of the hyperfine splitting in two-body atoms related to the HFS interval of the 1s state in muonium and positronium and the 2s state in hydrogen, deuterium and the helium-3 ion. We summarize their QED theory and pay special attention to involved effects of strong interactions and to recent optical measurements of the 2s HFS interval in hydrogen and deuterium. We present results for specific ratios of the 1s-2s frequencies in hydrogen and deuterium which happen to be among the most accurately measured and calculated quantities. 
S. Karshenboim, S. Eidelman, P. Fendel, V. G. Ivanov, N. Kolachevsky,   V. Shelyuto, T. W. Haensch