Vowel harmony and vowel alternation in Mayak

Like several other Western Nilotic languages, the Mayak variety of Northern Burun has two sets of vowels distinguished by the feature [ATR], the [-ATR] vowels [I, E, a, i, u] and the [+ATR] vowels [i, e, A, 0, u]. However, the mid [+ATR] vowels [e] and [o] are variants of the mid [-ATR] vowels /e/ and /i/ conditioned by a following high I+ATR] vowel. This allophony is the effect of one of four general vowel harmony processes. In addition, [-ATR) root vowels exhibit grammatically conditioned alternation which affects either [ATR] or height. The mixed character of this alternation invites the hypothesis that original mid [+ATR] vowels have merged with the high [-ATR] vowels, and this hypothesis is confirmed by a comparison of Mayak with other Western Nilotic languages.