We have conducted time-series photometric observations of the binary star V471 Tauri using the Whole Earth Telescope observing network. Our purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. Our observations have answered this question. Learning the cause of the white dwarf variations has been possible because of our discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, we have been able to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. We have found that U-band maximum coincides with X-ray minimum. From this result we have concluded that the magnetic rotator model accurately describes the variations we observe, but that models involving g-mode pulsations do not
