There is a book by Arthur Koestler, long out of print, which posits that the Kazars are the ancestors of the Ashkenazic Jews, having converted at a time when they were being squeezed between the Muslins and the Christians. They dispersed through Europe and are the ancestors of most of todays Hebrews [a word derived from Ivri, meaning Wanderer in Aramaic].

There is some wide belief in this theory, especially in Israel. In part, it let to the adoption of Sephardic spellings and pronunciation as purer than the Ashkenazic and therefore more appropriate for the language of the ]then new] Jewish state.