This photo (and the ones by FunkyGibbon) aren't C. erythrotis. They are probably a variant of C. ascanius whitesidei. There is a good side-by-side comparison illustration in Kingdon's Mammals of Afica, where this form is noted as an "unnamed monkey that may be a variant of the Congo Basin Red-tailed Monkey". They look very different to erythrotis.
The photo by @devilfish from Pata Zoo labelled as Red-eared Guenons also shows ascanius.
