1 Introduction

Among the early type stars of the main sequence various groups of chemically peculiar stars (henceforth CP stars) are found. According to the General Catalogue of Ap and Am stars (Renson et al. 1991), among the 6684 so far known or suspected CP stars, more than half (3427) are Am stars (CP1) the remaining 3257 being Bp or Ap. Among these only 190 stars belong to the He strong (82) or He weak (108) subgroups. Usually helium is underabundant in the coolest CP stars and overabundant in the hottest ones.

CP stars are characterised by spectral and photometric variations with a common period. In the oblique rotator model, proposed by Stibbs (1950), chemical elements are not homogeneously distributed on the stellar surface and the observed variations are due to the stellar rotation. Studying the photometric variability of six helium weak stars, Catalano & Leone (1996) found that the equivalent width of the HeI4026Å varies out of phase with respect to the photometric variability.

To investigate the behavior of helium and its relation to the spectral and light variability, we have performed time resolved spectroscopy of the HeI5876 Åline for a sample of 16 CP stars in the B6-A1 spectral range and belonging to different peculiarity classes.

  

Table 1: Average equivalent width $\langle$EW$\rangle$ and relative standard deviation $\sigma$ for the observed chemically peculiar stars. Spectral type and peculiarity class are from The General Catalogue of Ap and Am stars by Renson et al. (1991). $T_{\rm eff}$ and log g have been computed according to Moon & Dworetsky (1985) and Napiwotzki et al. (1993). N refers to the number of spectra, P(d) to the variability period in days with the relative reference. Source for V magnitudes and $v_{\rm e} \sin i$ was SIMBAD database. The "no line'' remark indicates the absence of the HeI5876Å line in the spectrum; "constant'', the absence of EW variations; "in phase'', the coincidence of maxima of the EW and photometric variations; and "out of phase'', the coincidence of maxima of the EW variation with the minima of photometric variations