2000A&A...355..315L

We have equipped the spectrograph of the Catania Astrophysical Observatory with a polarimetric module which gives simultaneous circularly right and left polarised radiation spectra. This facility has been used to perform time-resolved spectropolarimetric (Stokes V) measurements of the mean longitudinal (effective) magnetic field for seven chemically peculiar stars. Since this class of stars is characterised by a periodically variable magnetic field, the monitoring of the Stokes V parameter is a fundamental step to recover the magnetic field topology. To better define the variation of the effective magnetic field, we have combined our observations with data from the literature. Variability periods given in the literature have been verified using Hipparcos photometric data and, if necessary, we have re-determined them. From Hipparcos absolute magnitudes, we have determined the stellar radii and then, on the hypothesis of a rigid rotator, the inclination of the rotational axes with respect to the line of sight. On the hypothesis that the magnetic field presents a dominant dipolar component (that is, where the Stokes Q and U parameters are not necessary to recover the magnetic configuration) we have determined the angle between the rotational and dipole axes and the polar strength of the magnetic field. Chemically peculiar stars show periodic anti-phase light variations short-ward and long-ward of a constant wavelength, the null wavelength. We have performed numerical computations of the expected flux distribution for metal-enhanced atmospheres with different effective temperature and gravity. From the behaviour of the null wavelength, we confirm the importance of the non-homogeneous distribution of elements on the stellar surface as origin of the light variability. However to explain the photometric variability of some stars, we suggest that the flux distribution is also influenced by the contribution of the magnetic field to the hydrostatic equilibrium.