  Binary evolution theory predicts that accreting white dwarfs with sub-stellar
companions dominate the Galactic population of cataclysmic variables (CVs). In
order to test these predictions, it is necessary to identify these systems,
which may be difficult if the signatures of accretion become too weak to be
detected. The only chance to identify such "dead" CVs is by exploiting their
close binary nature. We have therefore searched the Sloan Digital Sky Survey
(SDSS) Stripe 82 area for apparently isolated white dwarfs that undergo
eclipses by a dark companion. We found no such eclipses in either the SDSS or
Palomar Transient Factory data sets among our sample of 2264 photometrically
selected white dwarf candidates within Stripe 82. This null result allows us to
set a firm upper limit on the space density, $\rho_0$, of dead CVs. In order to
determine this limit, we have used Monte-Carlo simulations to fold our
selection criteria through a simple model of the Galactic CV distribution.
Assuming a $T_{WD}=7,500$ K, the resulting 2$\sigma$ limit on the space density
of dead CVs is $\rho_0 \lesssim 2 \times 10^{-5}$ pc$^{-3}$, where $T_{WD}$ is
the typical effective temperature of the white dwarf in such systems.
