It is considered that a consecutive vowel "au" turned into a broad tone, and "ou", "oo" and "eu" turned into compound tones [oː] and [joː] respectively, but some researchers insist otherwise.
連続母音「あう（au）」は（開音）に、「おう（ou）」・「おお（oo）」と「えう（eu）」がそれぞれ、（合音）になったと考えられているが、異説もある。