According to tipster @mmddj_china who is based out of China, Samsung has started testing the Nougat update on multiple Galaxy C series devices. Specifically, Android 7.0 is being tested on the Galaxy C5, Galaxy C7, and the C9 Pro.
C5000ZCU1BQH2— 萌萌的电教 (@mmddj_china) August 10, 2017
C5 C7 C9 Pro Chinese version start testing Android7.0
He also notes that it is the Chinese variants of these phones. There is, however, no information on exactly when the roll out will begin. Given Samsung's track record, it may take sometime before the update arrives officially.