tokens: little variation of A e with rotation. The peak values are also quite different: 150 per cent for HD 37776, 50 per cent for HD 189775, and only 10 per cent for HD 175362. Stars with highly variable A e likely exhibit greater surface abundance anisotropies than stars in which A e is less variable. In order to obtain a single number with which to characterize the average strength of deviations between the magnetic curves derived for different elements for a given star, we used the idl curvefit routine to generate second-or third-order harmonic best fits, which were then integrated as functions of rotational phase. The significance of this integrated value is then A e /σ e , which is given for each star in the second-last column of Table 2 , where σ e is the mean uncertainty in the individual A e measurements. If A e /σ e 1, it can be concluded that differences in 〈 B z 〉 between different elements are a consequence of noise rather than real variations. It is this significance in which we are interested, as it will help to decide whether accurate measurements are available using metallic lines, or whether H lines should be used instead. Fig. 9 shows the histogram of A e /σ e for all stars. For the full sample (top panel), the median significance is 2.7σ. For the sub-sample of He-weak and He-strong Bp stars, the median of the distribution is at 2.9σ (second panel from the top). Amongst pulsating β Cep and SPB stars (third panel from the top) and the remainder of the sample with no apparent peculiarities (bottom panel), only one star, HD 96446, has A e /σ e ≥ 3: as HD 96446 is also a He-strong star, this likely reflects the star's chemical peculiarities. The medians of these distributions are 1.4σ and 1σ, respectively. While there are only eight non-chemically peculiar stars for which A e could be measured, these results are consistent with an origin of the effect in distortion due to chemical spots, and confirm that when chemical peculiarities are not present, there is no significant difference in 〈 B z 〉 when measured using different chemical elements. Even amongst the Bp stars, although there are several stars for which A e /σ e ≫ 3, differences in 〈 B z 〉 measured from different elements are negligible for many of the stars. Figure 9. Significance histogram for A e for (top–bottom) all stars in the sample for which A e could be measured, Bp stars, pulsating stars, and chemically normal, non-pulsating stars. The dashed line marks 3σ significance. Approximately half the sample shows at least a 3σ spread in 〈 B z 〉 measurements conducted with different elements. All of these are Bp stars: the only pulsator with A e /σ e ≥ 3 is HD 96446, which is also a He-strong star. It should be noted that while the mathematical treatment of A e is based upon random uncertainties, these variations are in fact systematic . Unfortunately, it is not obvious what simple mathematical tools might be utilized in order to capture the systematic variations between
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