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Princess Astra of Atrios was the sixth segment of the Key to Time.
The Princess was the sixth child of the sixth dynasty of the sixth royal house of Atrios, the last member of the line, which passed on the sixth segment of the Key to Time in their genetic structure. Astra was kidnapped by the Shadow and taken to the third planet. She was later transformed to her true shape. After the Fourth Doctor had defeated the Black Guardian, Princess Astra was restored to her humanoid form and returned to normal life. She ruled her planet and presumably married Merak. (TV: The Armageddon Factor)
Soon after Astra was restored, the Fourth Doctor's companion, Romana, regenerated and chose the appearance of Astra as her final form, saying that she liked the princess' appearance. (TV: Destiny of the Daleks)