More than a decade ago, German astronomer Reinhold Häfner was operating a telescope at the European Southern Observatory (ESO) when he saw a star suddenly vanish from his computer screen. Now, using newer ESO instruments, he has figured out exactly what happened.

The "star" that Häfner observed in 1988 actually is a system of two planet-sized stars. One, about twice the size of Earth, is an extremely hot, dense type of star called a white dwarf. The other, which is larger than Jupiter, is a significantly cooler and less massive star, called a red dwarf. Two-star systems are also known as binary systems.

Every three hours, the hot star disappears behind the cooler one, as seen from Earth. For several minutes, the brightness of the system drops by a factor of 250 and it becomes invisible to many telescopes.

The system in question is known as NN Serpentis, or NN Ser, due to its location in the constellation of Serpens. NN Ser's recurrent "disappearance" was long assumed to be an eclipse of one star by another, but it is only with the latest observations that the eclipse has been analyzed in detail. On June 10-11, Häfner monitored the system and determined that the eclipse is "total," meaning that the white dwarf is completely hidden as it moves behind the red dwarf. The observations also showed the stars are not far apart by astronomical standards, such that the entire system could fit easily inside our sun.