  The detection of pulsational frequencies in stellar photometry is required as
input for asteroseismological modelling. The second short run (SRa02) of the
CoRoT mission has provided photometric data of unprecedented quality and
time-coverage for a number of O-type stars. We analyse the CoRoT data
corresponding to three hot O-type stars, describing the properties of their
light curves and we search for pulsational frequencies, which we then compare
to theoretical model predictions. We determine the amplitude spectrum of the
data, using the Lomb-Scargle and a multifrequency HMM-like technique.
Frequencies are extracted by prewhitening, and their significance is evaluated
under the assumption that the light curve is dominated by red noise. We search
for harmonics, linear combinations and regular spacings among these
frequencies. We use simulations with the same time sampling as the data as a
powerful tool to judge the significance of our results. From the theoretical
point of view, we use the MAD non-adiabatic pulsation code to determine the
expected frequencies of excited modes. A substantial number of frequencies is
listed, but none can be convincingly identified as being connected to
pulsations. The amplitude spectrum is dominated by red noise. Theoretical
modelling shows that all three O-type stars can have excited modes but the
relation between the theoretical frequencies and the observed spectrum is not
obvious. The dominant red noise component in the hot O-type stars studied here
clearly points to a different origin than the pulsations seen in cooler O
stars. The physical cause of this red noise is unclear, but we speculate on the
possibility of sub-surface convection, granulation, or stellar wind
inhomogeneities being responsible.
