  We derive precise system parameters for the pre-cataclysmic binary, NN Ser.
  From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2
deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The
irradiation-induced emission lines from the surface of the secondary star give
a range of observed radial velocities. The corrected values give a radial
velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic
effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009
R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/-
0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} =
0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were
measured independently of any mass-radius relation. For the white dwarf, the
measured mass, radius and temperature show excellent agreement with a `thick'
hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius
of the secondary star is 10% larger than predicted by models, however,
correcting for irradiation accounts for most of this inconsistency, hence the
secondary star in NN Ser is one of the first precisely measured very low mass
objects to show good agreement with models. ULTRACAM r', i' and z' photometry
taken during the primary eclipse determines the colours of the secondary star
as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds
to a spectral type of M4 +/- 0.5. This is consistent with the derived mass,
demonstrating that there is no detectable heating of the unirradiated face,
despite intercepting radiative energy from the white dwarf which exceeds its
own luminosity by over a factor of 20.
