Twenty-year monitoring of the surface magnetic field of peculiar stars

Magnetic chemically peculiar stars of the main sequence can present rotational periods as long as many decades. Here we report the results of an observational campaign started in 2001 aimed to establish these very long periods from the variability of the surface magnetic field, as measured from the Zeeman split of the Fe{\sc ii}\,6149.258\,A spectral line. Thirty-six stars have been monitored with several high-resolution spectrographs at different telescopes, for a total of 412 new collected spectra. To extend and fill at the best the time frame, we have also exploited all public archives containing high-resolution spectra, many not yet published. At the start of the campaign, most of the variability periods were unknown and stars selected for the sharpness of their spectral lines only, and thirteen stars were found to present variability periods on the scale of weeks. A final plot of magnetic field strength versus rotational periods is given.