Context :

Aims : The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are examined in order to establish the properties of the non-radial pulsations in this star .

Methods : Fourier techniques are used to characterize the modulations of the He i \lambda \lambda 5876 , 6678 and H \alpha lines in several spectroscopic time series and to search for variations in a photometric time series .

Results : Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 hr .
The line profiles , especially those affected by emission wings , exhibit also modulations on longer time scales , but these are epoch-dependent and change from line to line .
Unlike previous claims , we find no unambiguous signature of the rotational period in our data , nor of a third pulsation period ( corresponding to a frequency of 2.66 d ^ { -1 } ) .

Conclusions : HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 hr with l \simeq 8 \pm 1 and l \simeq 4 \pm 1 respectively .
No significant signal is found in the first harmonics of these two periods .
The 2.89 hr mode is seen at all epochs and in all lines investigated , while the visibility of the 1.75 hr mode is clearly epoch dependent .
Whilst light variations are detected , their connection to these periodicities is not straightforward .