  The nearby star Procyon is a visual binary containing the F5 IV-V subgiant
Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B.
Using images obtained over two decades with the Hubble Space Telescope, and
historical measurements back to the 19th century, we have determined precise
orbital elements. Combined with measurements of the parallax and the motion of
the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun
and 0.592 +/- 0.006 Msun for A and B, respectively.
  The mass of Procyon A agrees well with theoretical predictions based on
asteroseismology and its temperature and luminosity. Use of a standard
core-overshoot model agrees best for a surprisingly high amount of core
overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr.
  Procyon B's location in the H-R diagram is in excellent agreement with
theoretical cooling tracks for white dwarfs of its dynamical mass. Its position
in the mass-radius plane is also consistent with theory, assuming a
carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was
1.9-2.2 Msun, depending on its amount of core overshoot.
  Several astrophysical puzzles remain. In the progenitor system, the stars at
periastron were separated by only ~5 AU, which might have led to tidal
interactions and even mass transfer; yet there is no direct evidence that these
have occurred. Moreover the orbital eccentricity has remained high (~0.40). The
mass of Procyon B is somewhat lower than anticipated from the
initial-to-final-mass relation seen in open clusters. The presence of heavy
elements in its atmosphere requires ongoing accretion, but the place of origin
is uncertain.
