  We present time-resolved optical spectrophotometry of the pulsating hydrogen
atmosphere (DA) white dwarf G 117-B15A. We find three periodicities in the
pulsation spectrum (215s, 272s, and 304s) all of which have been found in
earlier studies. By comparing the fractional wavelength dependence of the
pulsation amplitudes (chromatic amplitudes) with models, we confirm a previous
report that the strongest mode, at 215s, has l=1. The chromatic amplitude for
the 272s mode is very puzzling, showing an increase in fractional amplitude
with wavelength that cannot be reproduced by the models for any l at optical
wavelengths. Based on archival HST data, we show that while the behaviour of
the 215s mode at ultra-violet wavelengths is as expected from models, the weird
behaviour of the 272s periodicity is not restricted to optical wavelengths in
that it fails to show the expected increase in fractional amplitude towards
shorter wavelengths. We discuss possible causes for the discrepancies found for
the 272s variation, but find that all are lacking, and conclude that the nature
of this periodicity remains unclear.
