Context : 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 ( \sim 8.7 hours ) orbital period .
Its brightness and the presence of eclipses makes this system ideal for a more detailed study .

Aims : 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 .

Methods : 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 .

Results : The derived ephemeris is HJD _ { \mathrm { min } } = 2452784.5533 ( 1 ) + 0.3641394 ( 2 ) \times E. The red dwarf in DE~CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86 ^ { +3 \circ } _ { -2 } and the mass and radius of the red dwarf are 0.41 \pm 0.06 M _ { \odot } and 0.37 ^ { +0.06 } _ { -0.007 } R _ { \odot } , respectively , and the mass and radius of the white dwarf are 0.51 ^ { +0.06 } _ { -0.02 } M _ { \odot } and 0.0136 ^ { +0.0008 } _ { -0.0002 } R _ { \odot } , respectively .

Conclusions : 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 \leq 1.6 M _ { \odot } ) .
The current age of this system is 3.3 - 7.3 \times 10 ^ { 9 } years , while it will take longer than the Hubble time for DE~CVn to evolve into a semi-detached system .