Gau-Usu

 was a rocky planet in Mutter's Spiral. It was within a few dozen light-years of Earth.

The day on Gau-Usu lasted five Earth weeks. It had a ring system which looked like a monochromatic rainbow from the planet's surface. The Third Doctor visited the planet to help calibrate the TARDIS navigational computer. (PROSE: The Suns of Caresh)