Image caption An artist's conception of the white dwarf pair shows how they would radiate spirals of gravitational waves

Researchers have spotted visible-light evidence for one of astronomy's most elusive targets - gravitational waves - in the orbit of a pair of dead stars.

Until now, these ripples in space-time, first predicted by Einstein, have only been inferred from radio-wave sources.

But a change in the orbits of two white dwarf stars orbiting one another 3,000 light-years away is further proof of the waves that can literally be seen.

A study to be reported in Astrophysical Journal Letters describes the pair.

Gravitational waves were a significant part of Albert Einstein's general theory of relativity, which viewed space itself as a malleable construct, and the gravity of massive objects as a force that could effectively warp it.

Catching sight of an actual gravitational wave, however, is a tricky business; their effects tend to be tiny and the have so far eluded discovery in Earth-bound experiments.

But the wider Universe provides a laboratory in which the indirect effects of gravitational waves can be measured.

Six-second switch

In principle, any two massive objects orbiting one another can emit gravitational waves, slowly losing the momentum of their orbits into the waves.

The effect is to slightly change the size of the orbits, and the time it takes to complete them.

A measurement of a minuscule change in the orbits of rapidly rotating neutron stars called pulsars garnered the 1993 Nobel Prize in physics.

Image caption Much of the work was carried out at an optical telescope built in the 1930s

But pulling off the same trick with visible light has seemed unlikely until now.

It is the extreme nature of the pair of white dwarf stars known as J0651 - each a substantial fraction of our Sun's mass orbiting each other at a distance just a third that between the Earth and Moon - that increases the magnitude of the gravitational waves.

As members of the same team reported in Astrophysical Journal in 2011, the pair orbit each other in less than 13 minutes.

Since that discovery, the team has been keenly watching the pair "eclipse" one another, with each briefly blocking out the other's light as seen from Earth.

Over a period of 13 months, the team saw the orbital period reduce by less than a thousandth of a second, but the effect also shifts when the eclipse time is expected to happen, and that has shifted back by some six seconds since the pair were discovered.

"A lot of these indirect measurements have taken people years, mostly because the orbits are so much longer," explained lead author of the study, JJ Hermes from the University of Texas at Austin.

Mr Hermes told BBC News that he liked the idea that such a groundbreaking result was established in part by using a telescope nearly as old as Einstein's theory: the Otto Struve 2.1m telescope at the McDonald Observatory in Texas.

"There have been 30 years of using radio telescopes and timing pulsars, but this is the first time we've been able to detect the influence of gravitational wave radiation using an optical telescope," he told BBC News.

The team will continue to watch the pair's tightening orbit, and the expected eclipse time should shift back by another 20 seconds by next May. The results will also help guide observations from "direct detection" experiments.

These, such as the proposed eLisa project, aim to measure the tiny relative movements of vastly separated detectors as gravitational waves pass - a final, irrefutable proof that the waves are what relativity predicted so long ago.

"It would be a really nice confirmation if we got one of these laser interferometer missions going - we'd know exactly what to look for," Mr Hermes said.

"We've crunched the numbers, and eLisa would be able to detect this thing in about a week."