Napster has joined forces with XM Satellite Radio in the US, with the two companies testing a combined service.

The test is available to XM's five million US subscribers and lets them listen to music through the satellite radio service which they can then also buy and download through Napster.

Music purchased through the service will be playable on some future Windows-powered portable digital devices.

"XM + Napster greatly enhances the discovery aspect of XM by letting people instantly purchase and download the music they hear on XM Radio Online from Napster's catalogue," said Hugh Panero, president and CEO of XM Satellite Radio.