Ramarahasya Upanishad is one of the 31 Upanishads attached to the  Atharvaveda and classified as one of the 14 Vaishnava Upanishads. This is a late  Upanishad exclusively devoted to the Hindu god Rama. The text is largely recited by Hanuman, who states that Rama is identical to the supreme unchanging reality  Brahman, same as major Hindu deities, and the means to satcitananda and liberation. The text also includes sections on Tantra suggesting the beej mantra based on Rama.  The text enumerates many mantras, assigning them tantric powers, which it forms by 

combining root mantras. All these mantras are associated with Ram, but each formulation connotes a different role. The syllables in the mantras vary from 1 to  24 and evolve mantras like Om Ramaya hum phat swaha and Om srim Ram Dassarathaya  Sita vallabbhaya sarvabhita daya namah. Out of these mantras, those with two to six syllables are claimed by the text to be most beneficial.