The origin of the magnetic field in massive O-type stars is still under debate .
To model the physical processes responsible for the generation of O star magnetic fields , it is important to understand whether correlations between the presence of a magnetic field and stellar evolutionary state , rotation velocity , kinematical status , and surface composition can be identified .
The O4 Ief supergiant \zeta Pup is a fast rotator and a runaway star , which may be a product of a past binary interaction , possibly having had an encounter with the cluster Trumper 10 some 2 Myr ago .
The currently available observational material suggests that certain observed phenomena in this star may be related to the presence of a magnetic field .
We acquired spectropolarimetric observations of \zeta Pup with FORS 2 mounted on the 8-m Antu telescope of the VLT to investigate if a magnetic field is indeed present in this star .
We show that many spectral lines are highly variable and probably vary with the recently detected period of 1.78 d. No magnetic field is detected in \zeta Pup , as no magnetic field measurement has a significance level higher than 2.4 \sigma .
Still , we studied the probability of a single sinusoidal explaining the variation of the longitudinal magnetic field measurements .