  Context. We present time-resolved spectroscopy and photometry of HS
0218+3229, a new long-period cataclysmic variable discovered within the Hamburg
Quasar Survey. It is one of the few systems that allow a dynamical measurement
of the masses of the stellar components. Aims. We combine the analysis of
time-resolved optical spectroscopy and R-band photometry with the aim of
measuring the mass of the white dwarf and the donor star and the orbital
inclination. Methods. Cross-correlation of the spectra with K-type dwarf
templates is used to derive the radial velocity curve of the donor star. An
optimal subtraction of the broadened templates is performed to measure the
rotational broadening and constrain the spectral type of the donor. Finally, an
ellipsoidal model is fitted to the R-band light curve to obtain constraints
upon the orbital inclination of the binary system. Results. The orbital period
of HS 0218+3229 is found to be 0.297229661 +- 0.000000001 d (7.13351186 +-
0.00000002 h), and the amplitude of the donor's radial velocity curve is K2 =
162.4 +- 1.4 km/s. Modelling the ellipsoidal light curves gives an orbital
inclination in the range i = 59 +- 3 deg. A rotational broadening between 82.4
+- 1.2 km/s and 89.4 +- 1.3 km/s is found when assuming zero and continuum limb
darkening, respectively. The secondary star has most likely a spectral type K5
and contributes ~ 80-85% to the R-band light. Our analysis yields a mass ratio
of 0.52 < q < 0.65, a white dwarf mass of 0.44 < M1(Msol) < 0.65, and a donor
star mass of 0.23 < M2(Msol) < 0.44. Conclusions. We find that the donor star
in HS 0218+3229 is significantly undermassive for its spectral type. It is
therefore very likely that it has undergone nuclear evolution prior to the
onset of mass transfer.
