2000AJ....119.2382B

With Hubble Space Telescope Fine Guidance Sensor 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∼56,000 K. A weighted average with past parallax determinations (π_abs=14.6±0.4 mas) narrows the range of possible radius values, compared with past estimates. We obtain R_DA=0.0185±0.0008 R_☉ with uncertainty in the temperature and bolometric correction the dominant contributors to the error. Fine Guidance Sensor 3 photometry provides a light curve entirely consistent with reflection effects. A recently refined model mass-luminosity relation for low-mass stars provides a mass estimate for the M dwarf companion, M_dM=0.37±0.20 M_☉, where the mass range is due to metallicity and age uncertainties. Radial velocities from Vennes and Thorstensen provide a mass ratio from which we obtain M_DA=0.49^+0.19_–0.05 M_☉. Independently, our radius and recent logg determinations yield 0.44 M_☉<M_DA<0.47 M_☉. In each case, the minimum DA mass is that derived by Vennes & Thorstensen from their radial velocities and Keplerian circular orbits with i≤90°. Locating Feige 24 on an (M,R)-plane suggests a carbon core. Our radius and these mass estimates yield a value of γ_grav inconsistent with that derived by Vennes & Thorstensen. We speculate on the nature of a third component whose existence would resolve the discrepancy.