When one begins to examine the original shapes of the contemporary letter ы one cannot but pay attention to the fact that in both Slavic alphabets ы is rendered by two letters which are ъ followed ı and thus could be easily understood as a kind of ligature containing letter i like in ѥ [*je] and юу [*jau]. The problem is that none of the contemporary Slavic languages contains any traces of the i in reflexes of ы. George Shevelov (Shevelov 1964: 379) concludes in the section devoted to the second delabialization of rounded vowels that ’it is not excluded that certain Sl dialects, most plausibly in Balkans, could have had y with the phonetic value of an ui-diphthong. But it is impossible to pass definitive judgement on this and, if such dialects existed, to identify them geographically. The question must remain open.’
There is a plenty of place-names in Ingermanland whose contemporary Russian names contain ы. Comparing them with their Baltic Finnic counterparts allows me to investigate when the contacts between Eastern Slavic and Baltic Finnic might occur and, in particular, what were the predecessors of ы in the Slavic spoken in the area.
Shevelov, George 1964: A Prehistory of Slavic. Heidelberg, Carl Winter; Universitätsverlag