  We present here the analysis of an extensive set of high resolution optical
spectra of Plaskett's star (HD 47129). We use a disentangling method to
separate the individual spectra of each star. We derive a new orbital solution
and discuss the spectral classification of both components. A Doppler
tomography technique applied to the emission lines H alpha and He II 4686
yields a Doppler map that illustrates the wind interactions in the system.
Finally, an atmosphere code is used to determine the different chemical
abundances of the system components and the wind parameters. HD 47129 appears
to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage,
where matter has been transferred from the primary to the secondary star. The
He overabundance of the secondary supports this scenario. In addition, the N
overabundance and C underabundance of the primary component confirm previous
results based on X-ray spectroscopy and indicate that the primary is an evolved
massive star. Furthermore, the secondary star has a large rotational velocity
that deforms its surface, leading to a non-uniform distribution in effective
temperature. This could explain the variations in the equivalent widths of the
secondary lines with phase. We suggest that the wind of the secondary star is
confined near the equatorial plane because of its high rotational velocity,
affecting the ram pressure equilibrium in the wind interaction zone.
