Namkung, Ju. Phonological Inventories of Tibeto-Burman Languages. 1996. hh:hld:Namkung:Tibeto-Burman
"In native words, /p pj t ts tɕ k/ occur only with constricted vowels, while their
aspirated counterparts /ph phj th tsh tɕh kh/ occur only with unconstricted vowels."