Which Indian state was previously a kingdom, ruled by Palden Thondup Namgyal and his American wife Hope Cooke, before its annexation to India in 1975?

The Kingdom of Sikkim was a hereditary monarchy from 1642 to 1975 in the Eastern Himalayas. It was ruled by Chogyals (kings) of the Namgyal dynasty. Its 12th and the last king was Palden Thondup Namgyal who married Hope Cooke, which brought worldwide media attention to Sikkim. In 1975, as the result of a referendum, Sikkim became a state of India and the monarchy was abolished.