  Context. Plaskett's star (HD47129) is a very massive O-star binary in a post
Roche-lobe overflow stage. CoRoT observations of this system revealed
photometric variability with a number of frequencies. Aims. The aim of this
paper is to characterize the variations in spectroscopy and investigate their
origin. Methods. To sample its short-term variability, HD47129 was intensively
monitored during two spectroscopic campaigns of six nights each. The spectra
were disentangled and Fourier analyses were performed to determine possible
periodicities and to investigate the wavelength dependence of the phase
constant and the amplitude of the periodicities. Results. Complex line profile
variations are observed. Frequencies near 1.65, 0.82, and 0.37 d^-1 are
detected consistently in the He i 4471, He ii 4542, and N iii 4510-4518 lines.
These frequencies are consistent with those of the strongest signals detected
in photometry. The possibilities that these variations stem from pulsations, a
recently detected magnetic field or tidal interactions are discussed.
Conclusions. Whilst all three scenarios have their strengths, none of them can
currently account for all the observed properties of the line profile
variations.
