  We argue on the basis of astrometric and spectroscopic data that the
ultramassive white dwarf GD50 is associated with the star formation event that
created the Pleiades and is possibly a former member of this cluster. Its
cooling age (~60Myrs) is consistent with it having evolved essentially as a
single star from a progenitor with a mass M>6Msun so we find no need to invoke
a white dwarf-white dwarf binary merger scenario to account for its existence.
This result may represent the first direct observational evidence that single
star evolution can produce white dwarfs with M>1.1Msun, as predicted by some
stellar evolutionary theories. On the basis of its tangential velocity we also
provisionally identify the ultramassive (M~1.2Msun) white dwarf PG0136+251 as
being related to the Pleiades. These findings may help to alleviate the
difficulties in reconciling the observed number of hot nearby ultramassive
white dwarfs with the smaller number predicted by binary evolution models under
the assumption that they are the products of white dwarf mergers.
