On the basis of this result, we have started an observational campaign to search for infrared variability aimed to test the reliability of the common idea that the infrared region is not affected by all those phenomena that characterise the ultraviolet and visible parts of CP star spectra and to better understand the origin of the light variability, which is one of the outstanding observational aspects of some subgroups of CP stars.
In a previous paper (Catalano et al. 1997), the first in a series devoted to the investigation of the infrared variability of CP stars, we have shown that, out of the twenty observed CP2 stars of the subgroup SrEuCr, 19 have been found variable in the infrared and only one, i.e. HD 137949, was found to be non variable, at least on a time scale of a few years.
As it was stressed in Catalano et al. (1997), the variability of CP stars is currently explained in terms of the oblique rotator model, according to which, the period of the observed light, spectrum and magnetic field variations is nothing else than the rotational period, therefore accurate knowledge of the period of variability of CP stars is a fundamental step in understanding their complex behaviour, especially as far as it concerns the phase relation between the magnetic, spectral and light variations. Moreover the infrared variations of CP stars do show smaller amplitudes than in the visible, hence we mainly selected bright stars, not fainter than about 7.5 mag, and for which a period was previously known.
In this paper we report the results concerning twelve CP2 stars of the subgroup showing overabundances of Si and other elements.
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