Averaged, flux-calibrated spectrum of J180408 obtained at the San Pedro Mártir Observatory (Mexico) on Apr. 20, 2015 (MJD 57 132). The spectrum is not corrected for reddening. Solid lines indicate the positions of the most common hydrogen lines (H_α = 6563 Å and H_β = 4861 Å); dashed lines indicate the most prominent He I emission line positions (4713, 4921, 5015, 5876 and 6678 Å); finally, a dotted line indicates the wavelength of the He II line at 4686 Å. Two blue arrows indicate the tabulated position of the two suggested carbon lines (C II 4650 Å and C III 5810 Å). Inset: a zoom of the spectrum centred on the possibly detected He II line after the normalisation of the spectrum continuum to 1 is represented. A solid line indicates the position of the He II line (4686 Å).