Two ~35 day clocks in Hercules X-1: evidence for neutron star free precession R. Staubert, D. Klochkov, K. Postnov, N. Shakura, J. Wilms and R. E. Rothschild A&A, 494 3 (2009) 1025-1030 Published online: 22 December 2008 DOI: 10.1051/0004-6361:200810743