  We present time-resolved spectroscopy and photometry of SDSS
J100658.40+233724.4, which we have discovered to be an eclipsing cataclysmic
variable with an orbital period of 0.18591324 days (267.71507 min). The
observed velocity amplitude of the secondary star is 276 +/- 7 km/s, which an
irradiation correction reduces to 258 +/- 12 km/s. Doppler tomography of
emission lines from the infrared calcium triplet supports this measurement. We
have modelled the light curve using the LCURVE code and Markov Chain Monte
Carlo simulations, finding a mass ratio of 0.51 +/- 0.08. From the velocity
amplitude and the light curve analysis we find the mass of the white dwarf to
be 0.78 +/- 0.12 Msun and the masses and radii of the secondary star to be 0.40
+/- 0.10 Msun and 0.466 +/- 0.036 Rsun, respectively. The secondary component
is less dense than a normal main sequence star but its properties are in good
agreement with the expected values for a CV of this orbital period. By
modelling the spectral energy distribution of the system we find a distance of
676 +/- 40 pc and estimate a white dwarf effective temperature of 16500 +/-
2000 K.
