HIPPARCOS–Gaia DR2 parallax differences of old (red filled lozenges) and new (blue open lozenges) Ap stars as a function of the HIPPARCOS V magnitude (HPmag in Table 1). Error bars represent the sum of the errors from both catalogues. Two objects, for which the Gaia DR2 errors were larger than the HIPPARCOS ones, are labelled. Overplotted black dots mark stars that do not fulfil the last three astrometric criteria defined in Sect. 2.1. Magenta open hexagons and green open squares mark stars that were found to be binaries by Mathys (2017) and HIPPARCOS, respectively.