The sixth frequency of roAp star HR 1217

The frequency of the sixth mode observed in HR 1217 cannot currently be und erstood in the light of the asymptotic theory for high-frequency acoustic m odes, valid for adiabatic pulsations in spherically symmetric stars. Deviat ions from spherical symmetry will change the frequency of the observed osci llations away from the frequencies predicted by the asymptotic theory. Amon g other things, the presence of a magnetic field in the star will introduce such deviations from spherical symmetry. In the present work we inspect th e effect of the magnetic field on the oscillations of HR 1217. Although we model HR 1217 in a rather simplified manner, we believe, as argued in the t ext, that the essence of the conclusions would not be modified if a more so phisticated model were to be used instead. The results show that the freque ncy of the sixth mode observed in HR 1217 might be understood if the effect of the magnetic field on the frequencies of the oscillations is fully take n into account. Moreover, we interpret the sixth frequency as a mode of eve n degree, and explain the absence from the observations of an intermediate l = 1, m = 0 mode as the result of large energy losses associated with the coupling of the latter with Alfvenic waves, in the strongly magnetized oute r layers of the star. Finally, we show how, in the light of this explanatio n, the frequency of the sixth mode might be used in order to put constraint s on the parameters characterizing the star.