  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 of approximately 56,000 K degrees. A weighted average with
past parallax determinations (pi_abs = 14.6 +- 0.4 milliseconds of arc) narrows
the range of possible radius values, compared to past estimates. We obtain R_DA
= 0.0185 +- 0.0008 R_solar 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, M_dM = 0.37 +- 0.20
M_solar, 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 M_DA = 0.49+0.19-0.05 ~ M_solar. Independently, our radius and
recent log_g determinations yield 0.44 < M_DA < 0.47 M_solar. In each case the
minimum DA mass is that derived by Vennes & Thorstensen from their radial
velocities and Keplerian circular orbits with inclination <= 90 degrees.
Locating Feige 24 on an 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.
