Apsidal motion and light-time effect in eclipsing binaries HS Herculis and U Ophiuchi M. Wolf, P. Harmanec, R. Diethelm, K. Hornoch and P. Eenens A&A, 383 2 (2002) 533-539 Published online: 15 February 2002 DOI: 10.1051/0004-6361:20011729