Discovery of an Ultracool White Dwarf Companion

The discovery of a low luminosity common proper motion companion to the white dwarf GD392 at a wide separation of $46''$ is reported. $BVRI$ photometry suggests a low temperature ($T_{\rm eff}\sim4000$ K) while $JHK$ data strongly indicate suppressed flux at all near infrared wavelengths. Thus, GD392B is one of the few white dwarfs to show significant collision induced absorption due to the presence of photospheric ${\rm {H_2}}$ and the first ultracool white dwarf detected as a companion to another star. Models fail to explain GD392B as a normal mass white dwarf. If correct, the cool companion may be explained as a low mass white dwarf or unresolved double degenerate. The similarities of GD392B to known ultracool degenerates are discussed, including some possible implications for the faint end of the white dwarf luminosity function.