  The massive binary HDE 228766 is composed of an O type primary and an evolved
secondary. However, previous qualitative analyses of the composite spectrum
have led to a wide discussion about whether the secondary is an Of or a
Wolf-Rayet star. We use new observations and our novel QER20 package to
disentangle for the first time the spectra of the two stellar components and
obtain artefact free reconstructed spectra, yielding the more accurate and
reliable spectral classifications of O7.5 V((f))z for the primary and O6 Iaf
for the secondary. The emission features of the P-Cygni profiles of the
H\b{eta} and He I 5876 {\AA} lines, present in the reconstructed spectrum of
the secondary, show that this star is at an initial phase of its transition to
the WN evolutionary stage. A previously unobserved variable emission, composed
of at least four independent features, is seen since 2014 superposed to the
H{\gamma} absorption line. Our analysis reveals that these emission features
originate from a physically extended region. This could be explained by an
episode of enhanced mass-loss in the scenario of a non conservative evolution
of the binary.
