We present radio and submillimetre observations of the O4I ( n ) f star \zeta Pup , and discuss structure in the outer region of its wind ( \sim 10 - 100 ~ { } R _ { * } ) .
The properties of bremsstrahlung , the dominant emission process at these wavelengths , make it sensitive to structure and allow us to study how the amount of structure changes in the wind by comparing the fluxes at different wavelengths .
Possible forms of structure at these distances include Corotating Interaction Regions ( CIRs ) , stochastic clumping , a disk or a polar enhancement .
As the CIRs are azimuthally asymmetric , they should result in variability at submillimetre or radio wavelengths .
To look for this variability , we acquired 3.6 and 6 cm observations with the Australia Telescope Compact Array ( ATCA ) , covering about two rotational periods of the star .
We supplemented these with archive observations from the NRAO Very Large Array ( VLA ) , which cover a much longer time scale .
We did not find variability at more than the \pm 20 % level .
The long integration time does allow an accurate determination of the fluxes at 3.6 and 6 cm .
Converting these fluxes into a mass loss rate , we find \dot { M } = 3.5 \times 10 ^ { -6 } M _ { \sun } / \mathrm { yr } .
This value confirms the significant discrepancy with the mass loss rate derived from the H \alpha profile , making \zeta Pup an exception to the usually good agreement between the H \alpha and radio mass loss rates .
To study the run of structure as a function of distance , we supplemented the ATCA data by observing \zeta Pup at 850 \mu m with the James Clerk Maxwell Telescope ( JCMT ) and at 20 cm with the VLA .
A smooth wind model shows that the millimetre fluxes are too high compared to the radio fluxes .
While recombination of helium in the outer wind can not be discounted as an explanation , the wealth of evidence for structure strongly suggests this as the explanation for the discrepancy .
Model calculations show that the structure needs to be present in the inner \sim 70 ~ { } R _ { * } 
of the wind , but that it decays significantly , or maybe even disappears , beyond that radius .