We present the latest Hubble Space Telescope ( HST ) Space Telescope Imaging Spectrograph ( STIS ) E140M spectrum of the dwarf nova WZ Sge , obtained in July 2004 , 3 years following the early superoutburst of July 2001 .
This far-ultraviolet ( FUV ) spectrum covers the wavelength interval 1150-1725 Å , revealing Stark-broadened Ly \alpha absorption and absorption lines due to metals from a range of ionization states .
The Ly \alpha and C IV double peak emissions are still present , indicating the presence of an optically thin disk .
Single white dwarf synthetic spectral fits ( using \log { g } = 8.5 ) to the data indicate that the white dwarf has now reached a temperature T \approx 15 , 000 \pm 500 K .

Three years after the outburst the WD is still \sim 1500K above its quiescent temperature , it has an FUV flux level almost twice its pre-outburst value , and its spectrum does not distinctly exhibit the quasi-molecular hydrogen feature around 1400 Å which was present in the IUE and HST/GHRS pre-outburst data .
This is a clear indication that even three years after outburst the system is still showing the effect of the outburst .

Taking into account previous temperature estimates obtained during the earlier phase of the cooling , we model the cooling curve of WZ Sge , over a period of three years , using a stellar evolution code including accretion and the effects of compressional heating .
Assuming that compressional heating alone is the source of the energy released during the cooling phase , we find that ( 1 ) the mass of the white dwarf must be quite large ( \approx 1.0 \pm 0.2 M _ { \odot } ) ; and ( 2 ) the mass accretion rate must have a time-averaged ( over 52 days of outburst ) value of the order of 10 ^ { -8 } M _ { \odot } yr ^ { -1 } or above .
The outburst mass accretion rate derived from these compressional heating models is larger than the rates estimated from optical observations ( 24 ) and from a FUV spectral fit ( 20 ) by up to one order of magnitude .
This implies that during the cooling phase the energy released by the WD is not due to compressional heating alone .
We suggest that ongoing accretion during quiescence at a moderately low accretion rate can also release a significant amount of energy in the form of boundary layer irradiation , which can increase the temperature of the star by several thousand degrees .