The discovery of a frequency quintuplet and distorted dipole mode in the rapidly oscillating Ap star HD 6532

We have analysed 77 h of new, multisite observations of HD 6532 obtained in 1994, along with 20 h of observations from 1984 and 90 h from 1985. A frequency quintuplet with frequency separations equal to exactly the rotation frequency, and two harmonic frequencies, give a complete solution. The amplitude spectrum of the residuals shows white noise with its highest peaks of amplitude 0.1 mmag. We interpret the frequency quintuplet in terms of a distorted oblique dipole mode, which we model with a spherical harmonic series that is primarily dipolar with small radial and quadrupole contributions. This model does not fully describe the rotational modulation of pulsation amplitude and phase, which leads us to suggest that a completely decentred dipole model needs development. HD 6532 is only the second roAp star, after HR 3831, with a polarity-reversing pulsation mode which has been studied in sufficient detail to characterize the geometry of the mode.