Normalized spectra of O-type CSPN (see Table A.3). In these objects we could not detect the Balmer series. The interstellar absorption bands at λ4428, the complex at 5780 and 5890–6 are not indicated. The most important spectral features (absorption and emission) identified are: He II λ4200, 4542, and 4686, N V λ4604–19, for the blue spectral range (top panel), and O V λ5114, O VI λ5290, He II λ5412, O III λ5592, C III λ5696, C IV λ5801–12, for the red spectral range (bottom panel).