Hubble space telescope observations of white dwarfs in detached binaries

I describe three programs of Hubble Space Telescope (HST) observations of white dwarfs (WDs) in detached binaries. (1) I summarize the HST UV spectroscopy of the Hyades eclipsing binary V471 Tauri, containing a dK main-sequence star and a hot WD. By phasing time-resolved STIS spectra on the 9.25-min rotation period of the WD, we have shown that there are two opposite spots on the WD where there is magnetic accretion from the K star's wind. (2) Several programs of direct imaging and FGS astrometry are yielding high-precision visual orbits and dynamical masses, including the famous systems of Sirius and Procyon, as well as three fainter binaries in which both components are DC WDs. (3) Finally I discuss the bizarre nucleus of the low-surface-brightness planetary nebula EGB 6. The central star is associated with a compact emission-line nebula, which HST imaging shows is associated with a resolved dM companion. Why there should be a compact nebula around the cool star is puzzling; we speculate that it could be an accretion disk of material captured from the outflow that produced the surrounding faint PN. In a recent development, Spitzer has detected a 24 μm excess from EGB 6, indicating a dust component at a temperature of ∼260 K; the location of this dust relative to the hot and cool stars and the compact nebula remains uncertain at present.