Some like it cold?

It's the very model of a maladjusted marriage.

At first, both partners held their own. Little by little, however, the stronger drained the resources of the weaker, leaving it cold and underweight.

Astronomers came upon this odd couple in the course of observing the life cycles of pairs of stars that have been orbiting each other for 5 to 10 billion years. In one elderly pair, WZ Sagittae, a small but extremely dense star known as a white dwarf has stripped so much material from its average-density companion that this partner has only about 5 percent of the mass of the sun and a temperature of just 1,700 kelvins.

That's the coldest star on record, David R. Ciardi and Steve B. Howell of the University of Wyoming in Laramie and their colleagues will report in an upcoming Astrophysical Journal. By comparison, the visible surface of the sun has a temperature of 6,000 kelvins.

Although the white dwarf's partner remains a bona fide star, its mass is as low as that of a brown dwarf, an object that never acquired enough material to shine as stars do. Howell and his collaborators estimate that WZ Sagittae took at least 10 billion years to evolve to its present state, indicating that the pair's home galaxy, the Milky Way, must be at least that old.