  Blue horizontal-branch stars are Population II objects which are burning
helium in their core and possess a hydrogen-burning shell and radiative
envelope. Because of their low rotational velocities, diffusion has been
predicted to work in their atmospheres. In many respects, blue
horizontal-branch stars closely resemble the magnetic chemically peculiar stars
of the upper main sequence, which show photometric variability caused by
abundance spots on their surfaces. These spots are thought to be caused by
diffusion and the presence of a stable magnetic field. However, the latter does
not seem to be axiomatic. We searched for rotationally induced variability in
30 well-established bright field blue horizontal-branch stars in the solar
neighbourhood and searched the literature for magnetic fields measurements of
our targets. We employed archival photometric time series data from the ASAS,
ASAS-SN, and SuperWASP surveys. The data were carefully reduced and processed,
and a time series analysis was applied using several different techniques. We
also synthesized existing photometric and spectroscopic data of magnetic
chemically peculiar stars in order to study possible different surface
characteristics producing lower amplitudes. In the accuracy limit of the
employed data, no significant variability signals were found in our sample
stars. The resulting upper limits for variability are given. We conclude that
either no stellar surface spots are present in field blue horizontal-branch
stars, or their characteristics (contrast, total area, and involved elements)
are not sufficient to produce amplitudes larger than a few millimagnitudes in
the optical wavelength region. New detailed models taking into account the
elemental abundance pattern of blue horizontal-branch stars are needed to
synthesize light curves for a comparison with our results.
