  We present spectroscopy and photometry of GD 448, a detached white dwarf - M
dwarf binary with a period of 2.47h. We find that the NaI 8200A feature is
composed of narrow emission lines due to irradiation of the M dwarf by the
white dwarf within broad absorption lines that are essentially unaffected by
heating. Combined with an improved spectroscopic orbit and gravitational red
shift measurement from spectra of the H-alpha line, we are able to derive
masses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and
0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaII
emission lines to establish the radius of the M dwarf assuming the emission
from its surface to be proportional to the incident flux per unit area from the
white dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarf
appears to be a normal main-sequence star in terms of its mass and radius and
is less than half the size of its Roche lobe. The thermal timescale of the M
dwarf is much longer than the cooling age of the white dwarf so we conclude
that the M dwarf was unaffected by the common-envelope phase. The anomalous
width of the H-alpha emission from the M dwarf remains to be explained, but the
strengh of the line may be due to X-ray heating of the M dwarf due to accretion
onto the white dwarf from the M dwarf wind.
