We present high-speed , three-colour photometry of seven short period ( P _ { orb } \leq 95 mins ) eclipsing CVs from the Sloan Digital Sky Survey .
We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by \chi ^ { 2 } minimization .
Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum .
This is in line with predictions that 40–70 per cent of CVs should have evolved past the orbital period minimum .
Therefore , the main result of our study is that the missing population of post-period minimum CVs has finally been identified .
The donor star masses and radii are , however , inconsistent with model predictions ; the donor stars are approximately 10 per cent larger than expected across the mass range studied here .
One explanation for the discrepancy is enhanced angular momentum loss ( e.g .
from circumbinary discs ) , however the mass-transfer rates , as deduced from white dwarf effective temperatures , are not consistent with enhanced angular momentum loss .
We show it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity .
Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short period CVs .

The white dwarfs in our sample show a strong tendency towards high masses .
We show that this is unlikely to be due to selection effects .
The dominance of high-mass white dwarfs in our sample implies that erosion of the white dwarf during nova outbursts must be negligible , or even that white dwarfs grow in mass through the nova cycle .
Amongst our sample there are no Helium core white dwarfs , despite predictions that 30–80 per cent of short period CVs should contain Helium core white dwarfs .
We are unable to rule out selection effects as the cause of this discrepancy .