Observations of magnetic Ap stars at the Pic-du-Midi Observatory: From broadband linear polarization measurements with STERENN to spectropolarimetry with MuSiCoS

Observations of magnetic Ap stars taken at the 2m Telescope Bernard Lyot (TBL) of Pic-du-Midi Observatory are of the greatest importance in the study of the stellar magnetism. With the help of the polarimeter STERENN, in the '90s, Leroy and collaborators performed a vigorous campaign of observations of broadband linear polarization of Ap stars, virtually disclosing a new window on the transverse components of the stellar magnetic fields. Less than 10 years later, Donati, Landstreet, Wade and other collaborators are routinely using MuSiCoS in polarimetric mode to observe Stokes profiles of Ap and other kinds of magnetic stars. I will review the impact of TBL observations on the study of magnetic fields of Ap stars, and the progresses in the modelling that have been accomplished in the last ten years