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tokens: of 3.6 . The adopted width variation at each step corresponds to considering one more point at each side of the line profile, i.e. the minimal possible difference when the profile width is varied symmetrically around the centre. For each considered profile width, we performed a linear regression over the 50 synthetic spectra to obtain the optimal value of that gives the best results to determine H eff through equation ( 3 ). Note that it is not possible to obtain separately the values λ 0 and g 0 , but only their product. The results are shown in Fig. 2 . In the upper panel, we show the mean absolute percental error (MAPE) obtained by comparing the original values of H eff and the ones derived by the regressions, 1 while the lower panel shows the respective fitted values of , both as a function of the integration limits (width profiles). Figure 2. Mean absolute per centual error (upper panel) and normalization values (lower panel) as a function of the profiles width. In the right Y -axis of the lower panel, the values are normalized to the case when the profile width is 39.6 . The results were obtained analysing a sample of 50 synthetic spectra. The first remark of this test is that for all considered profile widths, it is always possible to fit a value of that allows us to infer very accurately H eff : the MAPE remains inferior to 1.0 per cent in the big majority of the cases. This result is quite unexpected since even underestimating the width of the profiles up to the extreme case in which the profiles consist of only five central points (from −3.6 to 3.6 ), one can obtain very precise estimations of H eff . Analogously, the same behaviour is obtained when the profiles are highly overestimated, with very small MAPE values. The fact that it is possible to consider deliberately only a part of the profiles to infer H eff can be useful in some practical applications, as, for example, in binary systems or in stars surrounded by circumstellar envelopes with strong stellar winds that can generate shocks visible as bumps. The bump produced by the shock is in turn blended with the intensity profile of the star (e.g. Sabin, Wade Lèbre 2015 ), so to consider only a fraction of the intensity profile of the star could be of interest. Another important result of this test is that it shows that the value of is very sensitive to the integration range, see lower panel of Fig. 2 . The fitted values of start at 329 nm (profile width of 7.2 ) and they increase very quickly to reach a maximum of 918 nm (profile width of 21.6 ), to then decrease following a likely exponential-type curve, finishing at 286 nm for the broadest profile width of 97.2 . Additionally, to illustrate the relative changes in the values of , we take as reference a width of 39.6 (from −19.8 to 19.8 ), which seems a plausible selection by visual inspection of the profiles in Fig. 1 . In the right Y -axis of the lower panel in Fig. 2 are shown the percentage variations of : as example, if