We have obtained FUSE and HST /STIS time-resolved spectroscopy of the polar AM Herculis during a deep low state .
The spectra are entirely dominated by the emission of the white dwarf .
Both the far-ultraviolet ( FUV ) flux as well as the spectral shape vary substantially over the orbital period , with maximum flux occurring at the same phase as during the high state .
The variations are due to the presence of a hot spot on the white dwarf , which we model quantitatively .
The white dwarf parameters can be determined from a spectral fit to the faint phase data , when the hot spot is self-eclipsed .
Adopting the distance of 79 \pm { 8 \atop 6 } pc determined by Thorstensen , we find an effective temperature of 19 800 \pm 700 K and a mass of \mbox { $M _ { \mathrm { wd } } $ } = 0.78 \pm { 0.12 \atop 0.17 } M _ { \odot } .
The hot spot has a lower temperature than during the high state , \sim 34 000 - 40 000 K , but covers a similar area , \sim 10 % of the white dwarf surface .
Low state FUSE and STIS spectra taken during four different epochs in 2002/3 show no variation of the FUV flux level or spectral shape , implying that the white dwarf temperature and the hot spot temperature , size , and location do not depend on the amount of time the system has spent in the low state .
Possible explanations are ongoing accretion at a low level , or deep heating – both alternatives have some weaknesses that we discuss .
No photospheric metal absorption lines are detected in the FUSE and STIS spectra , suggesting that the average metal abundances in the white dwarf atmosphere are lower than \sim 10 ^ { -3 } times their solar values .