  We present HST spectroscopy for 45 cataclysmic variables (CVs), observed with
HST/COS and HST/STIS. For 36 CVs, the white dwarf is recognisable through its
broad Ly$\alpha$ absorption profile and we measure the white dwarf effective
temperatures ($T_{\mathrm{eff}}$) by fitting the HST data assuming
$\log\,g=8.35$, which corresponds to the average mass for CV white dwarfs
($\simeq\,0.8\,\mathrm{M}_\odot$). Our results nearly double the number of CV
white dwarfs with an accurate temperature measurement. We find that CVs above
the period gap have, on average, higher temperatures ($\langle T_{\mathrm{eff}}
\rangle \simeq 23\,000\,$K) and exhibit much more scatter compared to those
below the gap ($\langle T_{\mathrm{eff}} \rangle \simeq 15\,000\,$K). While
this behaviour broadly agrees with theoretical predictions, some discrepancies
are present: (i) all our new measurements above the gap are characterised by
lower temperatures ($T_{\mathrm{eff}} \simeq 16\,000 - 26\,000\,$K) than
predicted by the present day CV population models ($T_{\mathrm{eff}} \simeq
38\,000 - 43\,000\,$K); (ii) our results below the gap are not clustered in the
predicted narrow track and exhibit in particular a relatively large spread near
the period minimum, which may point to some shortcomings in the CV evolutionary
models. Finally, in the standard model of CV evolution, reaching the minimum
period, CVs are expected to evolve back towards longer periods with mean
accretion rates $\dot{M}\lesssim 2 \times
10^{-11}\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$, corresponding to
$T_\mathrm{eff}\lesssim 11\,500\,$K. We do not unambiguously identify any such
system in our survey, suggesting that this major component of the predicted CV
population still remains elusive to observations.
