Example of line profile fitting of HD 228841 for six diagnostic lines of O-type stars (left panel). RV_PP estimates based on all presented lines for six stars (sorted by v sin i values) selected as illustrative cases: BD+36°4145 [O8.5 V(n)], BD+60°134 [O5.5 V(n)((f))], HD 228841 [O6.5 Vn((f))], HD 117490 [ON9.5 IIInn], HD 102415 [ON9 IV:nn], and ALS12370 [O6.5 Vnn((f))] (right panel). Note how, for those lines that are too weak, the cross-correlation technique provides individual RV measurements with a larger uncertainty (e.g. the O IIIλ5591 and He Iλ5015 lines in HD 228841, see left panel), or no results at all. While the He Iλ5875 line is always available and provides quite accurate RV_PP measurements, this is not the case for the other five diagnostic lines, for which the possibility to use them critically depends on the spectral type and v sin i combination.