  We present the first dynamical determination of the binary parameters of an
eclipsing SW Sextantis star in the 3-4 hour orbital period range during a low
state. We obtained time-resolved optical spectroscopy and photometry of HS
0220+0603 during its 2004-2005 low brightness state, as revealed in the
combined SMARTS, IAC80 and M1 Group long-term optical light curve.
  The optical spectra taken during primary eclipse reveal a secondary star
spectral type of M5.5 $\pm$ 0.5 as derived from molecular band-head indices.
The spectra also provide the first detection of a DAB white dwarf in a
cataclysmic variable. By modelling its optical spectrum we estimate a white
dwarf temperature of 30000 $\pm$ 5000 K.
  By combining the results of modelling the white dwarf eclipse from ULTRACAM
light curves with those obtained by simultaneously fitting the emission- and
absorption-line radial velocity curves and I-band ellipsoidal light curves, we
measure the stellar masses to be M$_1 = 0.87 \pm 0.09$ M$_\odot$ and M$_2 =
0.47 \pm 0.05$ M$_\odot$ for the white dwarf and the M dwarf, respectively, and
an inclination of the orbital plane of $i \approx 79^\mathrm{o}$. A radius of
$0.0103 \pm 0.0007$ R$_\odot$ is obtained for the white dwarf. The secondary
star in HS 0220+0603 is likely too cool and undersized for its mass.
