  We report a time-series analysis of the O4 I(n)fp star zeta Pup, based on
optical photometry obtained with the SMEI instrument on the Coriolis satellite,
2003--2006. A single astrophysical signal is found, with P = (1.780938 \pm
0.000093) d and a mean semi-amplitude of (6.9 \pm 0.3) mmag. There is no
evidence for persistent coherent signals with semi-amplitudes in excess of ca.
2~mmag on any of the timescales previously reported in the literature. In
particular, there is no evidence for a signature of the proposed rotation
period, ca. 5.1~days; zeta Pup is therefore probably not an oblique magnetic
rotator. The 1.8-day signal varies in amplitude by a factor ca. 2 on timescales
of 10--100d (and probably by more on longer timescales), and exhibits modest
excursions in phase, but there is no evidence for systematic changes in period
over the 1000-d span of our observations. Rotational modulation and
stellar-wind variability appear to be unlikely candidates for the underlying
mechanism; we suggest that the physical origin of the signal may be pulsation
associated with low-l oscillatory convection modes.
