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.
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