CCD-based observations of PG 0856+121 and a theoretical analysis of its oscillation modes A. Ulla, M. R. Zapatero Osorio, F. Pérez Hernández and J. MacDonald A&A, 369 3 (2001) 986-992 Published online: 15 April 2001 DOI: 10.1051/0004-6361:20010181