EUVE observations of ultra-massive white dwarfs

We have obtained extreme-ultraviolet spectra of five ultra massive white dwarfs detected during the EUVE and ROSAT-WFC all sky surveys. In most cases, these stars have mass between 1 and 1.3 M-circle dot which can be twice as large as the average mass of field white dwarfs. The origin of ultra-massive white dwarfs is somewhat uncertain although it has been suggested that they may result from the evolution of 8-11 M-circle dot stars, or the merger of two normal mass white dwarfs. Following the detection of helium in the extreme-ultraviolet spectrum of the ultra-massive white dwarf GD 50, we have observed several ultra-massive white dwarfs to determine whether or not presence of helium with an abundance like the one observed in GD 50 is typical. Our observations, so far, reveal no convincing cases of another GD 50 like ultra-massive white dwarf. Only the magnetic white dwarf PG 1659+44.0 may have some photospheric helium but with an abundance near the detection limit. We discuss the implications of the apparent lack of trace of helium in ultra-massive white dwarf atmospheres.