A star so strange it has sparked speculation about intelligent aliens is proving even more baffling to scientists.

Last year astronomers discovered that the star known as KIC 8462852 in the constellation Cygnus was sporadically dimming in a way that could not easily be explained.

Theories have ranged from an unusually large group of comets orbiting the star to a megastructure built by super-advanced aliens.

Now a new study from the Carnegie Institution in Washington DC, US, has deepened the mystery.

Further analysis of observations by the Kepler space telescope show that as well as displaying rapid brightness changes, the star has faded slowly and steadily over a period of four years.

Stars can appear dim if solid objects such as planets or dust clouds pass in front of them, but the erratic pattern shown by KIC 8462852 is unlike anything seen before.

When scientists conducted a detailed investigation of the steady fading data they were amazed to find that the star had dimmed by 2% in just six months while being watched by Kepler.

Astronomer Dr Ben Montet said: "The steady brightness change in KIC 8462852 is pretty astounding. Our highly accurate measurements over four years demonstrate that the star really is getting fainter with time. It is unprecedented for this type of star to slowly fade for years, and we don't see anything else like it in the Kepler data."

The Carnegie team believe the best explanation for the sudden dimming might be a collision or breakup of a planet or comet in the star's system, creating a short-term cloud of dust and debris.

However this would not account for the longer-term dimming observed during the first three years of the Kepler observations and also suggested by measurements dating back to the 19th century.

The research has been accepted for publication by The Astrophysical Journal.