Continuous monitoring of pulse period variations in Hercules X-1 using Swift/BAT D. Klochkov, R. Staubert, K. Postnov, N. Shakura and A. Santangelo A&A, 506 3 (2009) 1261-1267 Published online: 27 August 2009 DOI: 10.1051/0004-6361/200912791