Astronomers have described their discovery of four new hidden white dwarf stars "local" to Earth as a "surprise".

The University of Warwick team used the Hubble Space Telescope to pinpoint the small, dim stars orbiting in double systems within 65 light years of Earth.

The four white dwarf stars in this "nearby region" of space each have larger and brighter red dwarf companions that had previously concealed them.

University of Warwick research fellow Dr Mairi O'Brien said: "Nearby isolated white dwarfs are usually easy to find, but we couldn't see these four stars directly in visible wavelengths because their red dwarf companions were drowning out their light."

"It's a reminder that even in our own cosmic neighbourhood, we can still find surprises if we look in the right way, at the right wavelengths."

One of the stellar binaries, known as G203-47, is now officially the ninth closest white dwarf to the Sun.

The Warwick team, working with astronomers from the University of Colorado Boulder in the US, was interested in the four nearby systems because of a "substantial radial wobble".

This describes a phenomenon where a star wobbles back and forth, indicating a massive companion object is orbiting.

Co-author Dr David Wilson, research associate at the University of Colorado Boulder, explained their findings revealed some unusual activity.

For instance, G 203-47's red dwarf rotates once every 100 or so days, but orbits its white dwarf every 14.9 days, meaning they are not tidally synced.

"What's fascinating is that G 203-47 shouldn't be rotating this slowly if it formed the same way as similar systems," Wilson said.

"This suggests that these binaries have had very different evolutionary histories.

"Some underwent violent, prolonged interactions early on that locked them tidally. Others, like G 203-47, experienced gentler, briefer encounters that left them in this unusual state."

Prof Pier-Emmanuel Tremblay from the University of Warwick added the team thought there could be as many as nine or 10 additional local binary systems not yet discovered.

"If we put more targeted effort into observing red dwarfs, perhaps we will find more surprises like this," he said.

The findings have been published by the Royal Astronomical Society, external.