Summary
Lack of historical data has generated a few theories about the question as to who were holding the count's rights in Mid-Frisia between 1101, when the recently appointed Count Henry of Northeim was murdered, and 1138, when the Emperor granted the rights to the bishop of Utrecht. One of the theories holds that Otto of Rhieneck - married to one of the daughters of Henry of Northeim - inherited the rights.
The article below supports and amplifies this theory by using recently published numismatic evidence. This evidence suggests that the first successor of Henry of Northeim in the Mid-Frisian rights was his son Otto III. After Ottos' death in 1115/1117, he on his turn was probably succeeded by his brother in law, Otto of Rhieneck.