Profiles of the 4428 Å feature in IC 418 (in black) superposed on the profile of the O6.5 star HD 148937 (in blue). The O6.5 star has been displaced (in velocity) to try the 4428 Å band coincide with the profile and the minima of the 4428 Å band towards IC 418. Note that both spectra have been normalized on the EQW of the 4428 Å band. The 4428 Å profile in IC 418 is not fully reproduced by that in HD 148937 and its central wavelength is quite uncertain (see text).