Washington, Feb 13 (IANS): In a quest to find mismatched star pairs, known as extreme mass-ratio binaries, astronomers have discovered a new class of binary stars where one star is fully formed while the other is still in its infancy.

"We caught them at just the right time. In effect, we are seeing these stars in the delivery room," said lead author Maxwell Moe from the Harvard-Smithsonian Centre for Astrophysics (CfA) in Massachusetts.

The majority of stars in our galaxy come in pairs. In particular, the most massive stars usually have a companion.

These fraternal twins tend to be somewhat equal partners when it comes to mass - but not always.

The more massive a star is, the brighter it shines.

This makes it difficult to identify extreme mass-ratio binaries because the heavier star outshines, and thereby hides, the lighter star.

To combat this effect, Moe and his colleague Rosanne DiStefano looked for eclipsing systems in which the two stars line up in such a way that they periodically pass in front of each other as seen from Earth.

When the fainter star eclipses the brighter star, their combined light drops detectably.

These systems are rare because they require a precise alignment as seen from Earth.

After sifting through thousands of eclipsing systems, the team identified 18 extreme mass-ratio binaries in a neighbouring galaxy called the Large Magellanic Cloud.

The stars circle each other tightly in orbits with periods of three to nine days.

The more massive stars weigh six to 16 times as much as the Sun, while the less massive stars weigh about one to two times the Sun.

The discovery of these stellar twins could provide invaluable insight into the formation and evolution of massive stars, close binaries and star nurseries.