  Non-radial pulsations have been identified in a number of accreting white
dwarfs in cataclysmic variables. These stars offer insight into the excitation
of pulsation modes in atmospheres with mixed compositions of hydrogen, helium,
and metals, and the response of these modes to changes in the white dwarf
temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae
stands out by having a well-established observational record of three
independent pulsation modes that disappeared when the white dwarf temperature
rose dramatically following its 2007 accretion outburst. Our analysis of HST
ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations
produce variations in the white dwarf effective temperature as predicted by
theory. Additionally in May~2013, we obtained new HST/COS ultraviolet
observations that displayed unexpected behaviour: besides showing variability
at ~275s, which is close to the post-outburst pulsations detected with HST in
2010 and 2011, the white dwarf exhibits high-amplitude variability on a ~4.4h
time-scale. We demonstrate that this variability is produced by an increase of
the temperature of a region on white dwarf covering up to ~30 per cent of the
visible white dwarf surface. We argue against a short-lived accretion episode
as the explanation of such heating, and discuss this event in the context of
non-radial pulsations on a rapidly rotating star
