Walter spoke with an official at the German consulate in New York and found that all the relevant information is here. The applicable law reads: "Former German citizens who between January 30, 1933 and May 8, 1945 were deprived of their citizenship on political, racial, or religious grounds, and their descendants, shall on application have their citizenship restored."
The information page continues with the following explanation:
Between January 30, 1933 and May 8, 1945 there were basically two laws pertaining to the loss of German citizenship. With the 'Law on the Revocation of Naturalizations and the Deprivation of the German Citizenship' of July 14, 1933, some persons were deprived of their German citizenship individually. Their names were listed in the Reich Law Gazette ('Reichsgesetzblatt') and with the publication of the particular Reichsgesetzblatt they lost their German citizenship.I recently came across a listing of our Aunt Hilda, Elly's sister, having her citizenship revoked by name, but the second paragraph is the one that would apply to Elly Ringel and her daughter Helga. Walter writes that the official told him that if we have the proper documentation (and we do), then we could become citizens of Germany and of Europe within a year of filing the application.
The main group of former German citizens, however, lost their citizenship with the 'Eleventh Decree to the Law on the Citizenship of the Reich' of November 25, 1941. This stipulated that Jews living outside Germany could not be German citizens. This mainly affected Jews who had left Germany in the years before or shortly after the beginning of the Second World War.
What that means for any of us Rubys remains to be fully considered, but it is certainly interesting to understand that acquiring European citizenship (presumably without renouncing U.S. citizenship) is a realistic option for any of Helga's children and grandchildren.