  The light variability of hot magnetic chemically peculiar stars is typically
caused by the flux redistribution in spots with peculiar abundance. This raises
the question why some stars with surface abundance spots show significant
rotational light variability, while others do not. We study the Bp star HD
64740 to investigate how its remarkable inhomogeneities in the surface
distribution of helium and silicon, and the corresponding strong variability of
many spectral lines, can result in one of the faintest photometric
variabilities among the Bp stars. We used model atmospheres and synthetic
spectra calculated for the silicon and helium abundances from surface abundance
maps to predict the ultraviolet and visual light and line variability of HD
64740. The predicted fluxes and line profiles were compared with the observed
ones derived with the IUE, HST, and Hipparcos satellites and with spectra
acquired using the FEROS spectrograph at the 2.2m MPG/ESO telescope. We are
able to reproduce the observed visual light curve of HD 64740 assuming an
inhomogeneous distribution of iron correlated with silicon distribution. The
light variations in the ultraviolet are hardly detectable. We detect the
variability of many ultraviolet lines of carbon, silicon, and aluminium and
discuss the origin of these lines and the nature of their variations. The
maximum abundances of helium and silicon are not high enough to cause
significant light variations. The detected variability of many ultraviolet
lines is most likely of atmospheric origin and reflects the inhomogeneous
elemental surface distribution. The variability of the Civ resonance lines is
stronger and it probably results from the dependence of the wind mass-loss rate
on the chemical composition and magnetic field orientation. We have not been
able to detect a clear signature of the matter trapped in the circumstellar
clouds.
