Until recently it was generally believed that the properties that characterize Chemically Peculiar stars (or CP stars, according to Preston's 1974 scheme): peculiar abundances, magnetic fields, and so on, do not influence the longer wavelength part of the spectrum, since the present knowledge indicates the absence of significant flux redistribution and less important line blocking in this region than at shorter wavelengths (Muthsam & Weiss 1978; Hensberge & Van Rensbergen 1986). In fact, starting from the consideration that the spectral peculiarities of the CP stars make rather unreliable classical indirect methods for determining effective temperatures, Shallis & Blackwell (1979) used infrared fluxes as cornerstones of their method of integrated fluxes (Blackwell & Shallis 1977; Blackwell et al. 1979) which determines simultaneously effective temperature and angular diameters of stars.
Kroll et al. (1987) could finally show that the near infrared fluxes and colors of CP stars, when compared to a black body, are normal, like that of early main sequence stars. IRAS data could even prove that the normality of IR fluxes is guaranteed to at least 25 (Kroll 1987): only two CP4 stars showed flux excesses longward of 60, showing cold circumstellar material, which is not uncommon among early B stars.
Moreover Leone & Catalano (1991) have shown that the solar composition Kurucz model atmospheres, which are used to fit the CP stars spectra from 5500 to 16500 Å, give a fair representation of the overall flux distribution, with the exception of the Balmer region, where CP stars appear generally brighter than normal stars, this excess being just a few percent of the total flux.
In spite of this normality of the infrared behavior, peculiar abundances and/or magnetic fields seem to affect the near infrared. In fact, Catalano et al. (1991, hereafter CKL) have shown that, out of the eight CP stars monitored throughout their rotational periods, at least six are variable in the near infrared, although the amplitudes shown are smaller than in the visible.
In order to test the validity of common idea that the infrared region is not affected by all those phenomena that characterize the ultraviolet and visible parts of CP2 stars spectra, we have started an observational campaign to search for infrared variability, also in order to understand better the origin of the light variability, which is one of the outstanding observational aspects of these stars.
In this paper we report the results concerning twenty CP2 stars of the subgroup showing overabundances of Sr, Cr, and/or Eu.