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We present spectroscopy and photometry of GD 448, a detached white dwarf - Mdwarf binary with a period of 2.47h. We find that the NaI 8200A feature iscomposed of narrow emission lines due to irradiation of the M dwarf by thewhite dwarf within broad absorption lines that are essentially unaffected byheating. Combined with an improved spectroscopic orbit and gravitational redshift measurement from spectra of the H-alpha line, we are able to derivemasses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaIIemission lines to establish the radius of the M dwarf assuming the emissionfrom its surface to be proportional to the incident flux per unit area from thewhite dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarfappears to be a normal main-sequence star in terms of its mass and radius andis less than half the size of its Roche lobe. The thermal timescale of the Mdwarf is much longer than the cooling age of the white dwarf so we concludethat the M dwarf was unaffected by the common-envelope phase. The anomalouswidth of the H-alpha emission from the M dwarf remains to be explained, but thestrengh of the line may be due to X-ray heating of the M dwarf due to accretiononto the white dwarf from the M dwarf wind.Comment: 8 pages, 8 figure

The mass and radius of the M dwarf companion to GD 448