  Close white dwarf - red dwarf binaries must have gone through a
common-envelope phase during their evolution. DE CVn is a detached white dwarf
- red dwarf binary with a relatively short (~8.7 hours) orbital period. Its
brightness and the presence of eclipses makes this system ideal for a more
detailed study. From a study of photometric and spectroscopic observations of
DE CVn we derive the system parameters which we discuss in the frame work of
common-envelope evolution. Photometric observations of the eclipses are used to
determine an accurate ephemeris. From a model fit to an average low-resolution
spectrum of DE CVn we constrain the temperature of the white dwarf and the
spectral type of the red dwarf. The eclipse light curve is analysed and
combined with the radial velocity curve of the red dwarf determined from
time-resolved spectroscopy to derive constraints on the inclination and the
masses of the components in the system. The derived ephemeris is HJD_min =
2452784.5533(1) + 0.3641394(2) x E. The red dwarf in DE CVn has a spectral type
of M3V and the white dwarf has an effective temperature of 8000 K. The
inclination of the system is 86 (+3, -2) deg and the mass and radius of the red
dwarf are 0.41 +/- 0.06 M_sun and 0.37 (+0.06, -0.007) R_sun, respectively, and
the mass and radius of the white dwarf are 0.51 (+0.06, -0.02) M_sun and 0.0136
(+0.0008, -0.0002) R_sun, respectively. We found that the white dwarf has a
hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a
common-envelope phase, we can reconstruct its evolution and we find that the
progenitor of the white dwarf was a relatively low-mass star (M <= 1.6M_sun).
The current age of this system is 3.3-7.3 x 10^9 years, while it will take
longer than the Hubble time for DE CVn to evolve into a semi-detached system.
