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 \pm 0.000000001 d ( 7.13351186 \pm 0.00000002 h ) , and the amplitude of the donor ’ s radial velocity curve is K _ { 2 } = 162.4 \pm 1.4 \mathrm { km~ { } s ^ { -1 } } .
Modelling the ellipsoidal light curves gives an orbital inclination in the range i = 59 ^ { \mathrm { o } } \pm 3 ^ { \mathrm { o } } .
A rotational broadening between 82.4 \pm 1.2 \mathrm { km~ { } s ^ { -1 } } and 89.4 \pm 1.3 \mathrm { km~ { } s ^ { -1 } } is found when assuming zero and continuum limb darkening , respectively .
The secondary star has most likely a spectral type K5 and contributes \sim 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 < M _ { 1 } ( M _ { \odot } ) < 0.65 , and a donor star mass of 0.23 < M _ { 2 } ( M _ { \odot } ) < 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 .