Two ~35 day clocks in Hercules X-1: evidence for neutron star free precessionR. Staubert, D. Klochkov, K. Postnov, N. Shakura, J. Wilms and R. E. RothschildA&A, 494 3 (2009) 1025-1030DOI: https://doi.org/10.1051/0004-6361:200810743
