Hewa phonemes: a tentative statement . Vollrath, Paul W. (1985) · In Five phonological studies edited by Clifton, John M. · Summer Institute of Linguistics

Ethnologue: Languages of the World, 18th Edition . (2015) · edited by Lewis, M. Paul, Gary F. Simons, and Charles D. Fennig · SIL International http://www.ethnologue.com

Sepik languages: checklist and preliminary classification . Laycock, Donald C. (1973) · Pacific Linguistics · edited by Wurm, Stephen A. · Australian National University, Research School of Pacific Studies, Department of Linguistics

Glottolog 2.3 . Hammarström, Harald & Forkel, Robert & Haspelmath, Martin & Nordhoff, Sebastian (2014) · http://glottolog.org