  G39-27/289 is a common proper motion pair formed by a white dwarf
(WD0433+270) and a main-sequence star (BD+26 730) that apparently has been
classified as a member of the Hyades open cluster. Previous studies of the
white dwarf component yielded a cooling time of ~4 Gyr. Although it has not
been pointed out before explicitly, this result is 6 times larger than the age
of the Hyades cluster, giving rise to an apparent conflict between the physics
of white dwarfs and cluster main-sequence fitting. We investigate whether this
system belongs to the Hyades cluster and, accordingly, give a plausible
explanation to the nature of the white dwarf member. From kinematic and
chemical composition considerations we believe that the system was a former
member of the Hyades cluster and therefore has an evolutionary link with it.
However, the evidence is not conclusive. With regards to the nature of the
white dwarf component, we find that two core compositions (C/O and Fe) are
compatible with the observed effective temperature and radius. These
compositions yield very different cooling times of ~4 Gyr and ~1 Gyr,
respectively. We distinguish two posssible scenarios. If the pair does not
belong to the Hyades cluster but only to the Hyades stream, this would indicate
that such stream contains rather old objects and definitely not coeval with the
cluster. This has interesting consequences for Galactic dynamics. On the
contrary, if the white dwarf has an Fe core, it will have a cooling time
compatible with the Hyades age. This is a tantalizing result that would have
implications for the thermonuclear explosion of white dwarfs and explosion
theories of degenerate nuclei.
