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The light variability of hot magnetic chemically peculiar stars is typicallycaused by the flux redistribution in spots with peculiar abundance. This raisesthe question why some stars with surface abundance spots show significantrotational light variability, while others do not. We study the Bp star HD64740 to investigate how its remarkable inhomogeneities in the surfacedistribution of helium and silicon, and the corresponding strong variability ofmany spectral lines, can result in one of the faintest photometricvariabilities among the Bp stars. We used model atmospheres and syntheticspectra calculated for the silicon and helium abundances from surface abundancemaps to predict the ultraviolet and visual light and line variability of HD64740. The predicted fluxes and line profiles were compared with the observedones derived with the IUE, HST, and Hipparcos satellites and with spectraacquired using the FEROS spectrograph at the 2.2m MPG/ESO telescope. We areable to reproduce the observed visual light curve of HD 64740 assuming aninhomogeneous distribution of iron correlated with silicon distribution. Thelight variations in the ultraviolet are hardly detectable. We detect thevariability of many ultraviolet lines of carbon, silicon, and aluminium anddiscuss the origin of these lines and the nature of their variations. Themaximum abundances of helium and silicon are not high enough to causesignificant light variations. The detected variability of many ultravioletlines is most likely of atmospheric origin and reflects the inhomogeneouselemental surface distribution. The variability of the Civ resonance lines isstronger and it probably results from the dependence of the wind mass-loss rateon the chemical composition and magnetic field orientation. We have not beenable to detect a clear signature of the matter trapped in the circumstellarclouds.

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740