With HST FGS 3 we have determined a parallax for the white dwarf - M dwarf interacting binary , Feige 24 .
The white dwarf ( DA ) component has an effective temperature , T _ { eff } \sim 56 , 000 K .
A weighted average with past parallax determinations ( \pi _ { abs } = 14.6 \pm 0.4 milliseconds of arc ) narrows the range of possible radius values , compared to past estimates .
We obtain R _ { DA } = 0.0185 \pm 0.0008 R _ { \sun } with uncertainty in the temperature and bolometric correction the dominant contributors to the error .
FGS photometry provides a light curve entirely consistent with reflection effects .
A recently refined model Mass-Luminosity Relation ( Baraffe et al .
1998 ) for low mass stars provides a mass estimate for the M dwarf companion , { \cal M } _ { dM } = 0.37 \pm 0.20 { \cal M } _ { \sun } , where the mass range is due to metallicity and age uncertainties .
Radial velocities from Vennes and Thorstensen ( 1994 ) provide a mass ratio from which we obtain { \cal M } _ { DA } = 0.49 ^ { +0.19 } _ { -0.05 } ~ { } { \cal M } _ { \sun } .
Independently , our radius and recent log~ { } g determinations yield 0.44 < { \cal M } _ { DA } < 0.47 { \cal M } _ { \sun } .
In each case the minimum DA mass is that derived by Vennes & Thorstensen from their radial velocities and Keplerian circular orbits with i \leq 90 \deg .
Locating Feige 24 on an { \cal M } - R plane suggests a carbon core .

Our radius and these mass estimates yield a \gamma _ { grav } inconsistent with that derived by Vennes & Thorstensen .
We speculate on the nature of a third component whose existence would resolve the discrepancy .