Normalised spectra (black curves) of our OB star sample. Tickmarks indicate the expected position of the hydrogen lines (black solid), the diffuse insterstellar absorption band (black dash-dot-dot), the helium I (red dash-dot-dot) and helium II (long red dash-dot-dot) lines, and the CIII-4069 and NIII-4097 lines (blue dash-dot-dot). The red plot is the template corresponding to the indicated spectral type.