2248 20160706142859.0 DOI 10.1051/0004-6361/201117261 ASTROimport-405 Hubrig, S. 2011 Aims: Only for very few β Cephei stars has the behaviour of the magnetic field been studied over the rotation cycle. During the past two years we have obtained multi-epoch polarimetric spectra of the β Cephei star V1449 Aql with SOFIN at the Nordic Optical Telescope to search for a rotation period and to constrain the geometry of the magnetic field. Methods: The mean longitudinal magnetic field is measured at 13 different epochs. The new measurements, together with the previous FORS 1 measurements, have been used for the frequency analysis and the characterization of the magnetic field. Results: V1449 Aql so far possesses the strongest longitudinal magnetic field of up to 700 G among the β Cephei stars. The resulting periodogram displays three dominant peaks with the highest peak at f = 0.0720 d-1 corresponding to a period P = 13ḍ893. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength Bd around 3 kG and an inclination angle β of the magnetic axis to the rotation axis of 76 ± 4°. As of today, the strongest longitudinal magnetic fields are detected in the β Cephei stars V1449 Aql and ξ1 CMa with large radial velocity amplitudes. Their peak-to-peak amplitudes reach ~90 km s-1 and ~33 km s-1, respectively. Concluding, we briefly discuss the position of the currently known eight magnetic β Cephei and candidate β Cephei stars in the Hertzsprung-Russell (H-R) diagram. Based on observations obtained at the 2.56-m Nordic Optical Telescope on La Palma and ESO Prgs. 077.D-0311 and 178.D-0361.Appendix A is available in electronic form at http://www.aanda.org The strong magnetic field of the large-amplitude ß Cephei pulsator V1449 Aquilae Ilyin, I. Briquet, M. Schöller, M. González, J. F. Nuñez, N. De Cat, P. Morel, T. L20 Astronomy and Astrophysics 531 2011 http://esoads.eso.org/abs/2011A%26A...531L..20H published in REFERD