Using the UK Infrared Telescope (UKIRT) in Hawaii, astronomers have discovered two examples of a star type that had been predicted but never observed. Estimates place the Jupiter-size stars between 100 and 130 light-years away. Steve B. Howell of the Planetary Science Institute in Tucson, Ariz., and his colleagues describe their findings in a report to be published in the Astrophysical Journal Letters.

Though they resemble brown dwarfs, these small, cool stars are actually vestiges of ordinary stars that have dwindled over time, donating material to their white dwarf companions. The two donor-and-recipient pairs represent two different binary star systems. One, known as LL Andromedae, is a so-called dwarf nova. In this system, material spilling from the donor star, or "cool dwarf secondary," forms an accretion disk around the recipient white dwarf, as in the artist's conception shown here. The second pair, EF Eridani, is classified as a "polar." White dwarfs in this system are strongly magnetic. Material from the donor star thus flows directly onto the magnetic poles of the white dwarf, instead of forming an accretion disk.

More research will be needed to unlock the secrets of the new stars, which, at about eight billion years old, are as ancient as the galaxy itself. Despite their similarities to brown dwarfs in terms of temperature, size and mass, Howell notes, their structure and composition is probably different.