Mean longitudinal magnetic field (top left), crossover (bottom left), and mean magnetic field modulus (bottom right) of the star HD 188041, against rotation phase. The longitudinal field measurements of the present paper and those of Mathys (1994) and Mathys & Hubrig (1997) have been arbitrarily shifted by −400 G for consistency with the more numerous data of Babcock (1954, 1958) (asterisks). Except for the latter, the symbols are as described at the beginning of Appendix A. The mean quadratic field could not be determined in this star (see text for details).