Using the Green Bank Telescope, the world’s largest fully steerable radio telescope, Kaplan et al. (2014) made observations of the pulsar PSR J2222-0137 and its invisible companion. A pulsar is basically a neutron star, an ultra-dense remnant core of a massive star. The observations show that PSR J2222-0137 is 1.20 ± 0.14 times the Sun’s mass and its invisible companion is 1.05 ± 0.06 times the Sun’s mass. Both objects circle around each other with an orbital period of 2.45 days. The pulsar’s invisible companion tugs at the pulsar, allowing its presence to be inferred.
Based on its estimated mass, the pulsar’s invisible companion is not massive enough to be a black hole. Furthermore, the companion’s orbit is too circular for it to be another neutron star since neutron stars form in violent supernova explosions that would leave behind a system with a high orbital eccentricity. Despite deep optical and near-infrared searches, the pulsar’s companion remains invisible. This suggests the companion is probably a very old and very cool white dwarf with a surface temperature of less than 3000K. With such a temperature, it would be far cooler than any known white dwarf and would not be directly detectable. The invisible white dwarf companion of PSR J2222-0137 may have cooled enough to have already crystallised and is now in the faster Debye cooling regime. In this cooling regime, the crystal lattice of the white dwarf causes coherent vibration which allows the white dwarf to cool more rapidly.
Kaplan et al. (2014), “A 1.05 M⊙ Companion to PSR J2222-0137: The Coolest Known White Dwarf?”, arXiv:1406.0488 [astro-ph.SR]