Astronomers have found a weird double-star system -- a super-fast spinning pulsar whose gravity has deformed its companion star into a giant red teardrop.

The discovery was made by astronomers from the Astronomical Observatory of Bologna and their international colleagues, using the combined powers of the Parkes radio telescope and the Hubble Space Telescope.

"Such a system has never been seen before," says Dr Andrea Possenti of the Astronomical Observatory of Bologna, a member of the observing team.

The astronomers may be seeing the system in a fleeting phase of its life -- the point at which a new millisecond pulsar has just been fully "spun up" by its companion star.

But it's also possible that the pulsar has picked up the red star after parting from its original partner.

Pulsars are small spinning neutron stars, the super-dense cores of massive stars that have exploded. They are only the size of a small city. Pulsars emit beams of radio waves: as the beam from the spinning star flashes across the Earth, it appears as a tell-tale "pulse."

"Normal" pulsars spin at up to a few times a second, but the super-fast "millisecond" pulsars spin at hundreds of times a second.

The pulsar in question, called J1740-5340, spins 274 times a second. It was found in 2000 by an international team using the Parkes telescope to search for millisecond pulsars.

The searchers targeted 96 "globular clusters" -- tightly packed balls of stars on the edge of our Galaxy that are known breeding grounds for millisecond pulsars. J1740-5340 was found in a globular cluster called NGC 6397.

Dr. Nichi D'Amico (Astronomical Observatory of Bologna) and his colleagues analyzed the pulsar's signal and found that it was hidden about half the time by matter from the companion star, which the pulsar orbits in 1.35 days. Even when present, the peek-a-boo signal showed signs of having struggled through a cloud of gas from the companion.

The erratic radio signals told the observers they had something unusual on their hands. Most of the 90 known millisecond pulsars waltz through space with partner stars, but in almost all cases the partner is a "white dwarf" -- an old, shrunken star at the end of its life. But white dwarfs are tiny objects, not big enough to eclipse a pulsar's signal for half of its orbit.

So Dr. Francesco Ferraro and collaborators from the Astronomical Observatory of Bologna hunted through archival data from the Hubble Space Telescope and the European Southern Observatory to identify the companion.

Sitting near the pulsar they found an unusually red star. Its shape and brightness seem to fluctuate in step with the pulsar's orbit. This fits with the idea of the star being drawn out into a long teardrop by the pulsar: as the pulsar orbits, it drags the star around. The angle at which we are viewing the elongated star is constantly changing, which changes the star's appearance.

The team concluded that the red star had filled and overflowed its "Roche lobe," a teardrop-shaped envelope in space which defines where matter can remain held to the star by gravity. This is the first system in which a millisecond pulsar has been seen with a companion that completely fills its Roche lobe.

Astronomers think millisecond pulsars are old, slow pulsars that have been recycled. The pulsar's gravity sucks off gas from a larger companion star. As the gas hits the pulsar surface, it transfers energy to it, making it spin faster. The result is a turbocharged pulsar spinning at hundreds of times a second. Meanwhile, the enfeebled companion has dwindled into a white dwarf.

But how does a millisecond pulsar with a bloated red companion fit this picture?

"We could be seeing the system at a very special phase of its life," says Dr. Possenti. "That is, the pulsar has acquired enough gas from its companion to be spinning at hundreds of times a second, but the companion star has not yet shrunken into a white dwarf."

But the red star could also be an interloper. As the pulsar jostled through the throng of stars in the globular cluster, it may have swapped its original white-dwarf partner for the red star. Such interactions are common in globular clusters, where the density of stars is a million times greater than in our Sun's neighborhood.