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H.E. Bond, R. Gilliland (STScI), K.G. Schaefer (Towson U. \& STScI), T. Girard (Yale U.)
\hyphenation{Sirius} \def\secpoint{''\mskip-7.6mu.\,}
Only a handful of white dwarfs have directly measured dynamical masses, including those in the well-known visual binaries 40~Eri, Sirius, and Procyon. Using the Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope, we have begun a long-term program of direct imaging of two visual binaries containing white dwarfs.
Procyon. This system consists of an F5 IV-V star and a very faint 8700~K DZ white-dwarf companion. Its separation and position angle are extremely difficult to measure from the ground, due to the large brightness difference of about 8~mag. We combine short (0.12~s) and long (100~s) WFPC2 exposures without moving the telescope, and are monitoring Procyon once a year. Our existing observations already show that the ground-based measurements of the separation are seriously in error, leading to downward revisions of the masses of both components (A: 1.46 M\odot; B: 0.59 M\odot), and removing a long-standing conflict between the observed and theoretically predicted luminosity of Procyon~A.
G 107-70. G 107-70 consists of two cool DC white dwarfs, whose separation of about 0\secpoint7 makes it difficult to measure from the ground. We are monitoring it twice a year, in order eventually to determine dynamical masses for both white dwarfs. The existing observations have already revised the orbital period to a value near 18.67~yr.
We are also observing {\bf\mu~Cassiopeiae} twice a year. This binary consists of a metal-poor G dwarf and a faint dM companion. Direct WFPC2 imaging at 9500~Å\ easily reveals the companion. Eventually we will be able to determine the helium content of the system, from the mass-luminosity relation.
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