Sachsen-Altenburg Naturalization and Citizenship
In Germany as well as in the Kingdom of Saxony, authorities kept the so called Bürgerbücher or Bürgerrollen (lists of people who lived in cities, who had been given the right to live in a city (Neubürger) because they paid a sum of money for the privilege ( the sum was lower when a new citizen married a girl or a widow from a particular city). They now had the privilege to practise a profession, they had voting rights, but they also had to pay taxes and defend the city. Well into the 19th century such books are a well-documented source in which one can find where the new citizens originated from. Bürgerbücher are the invention of cities, therefore, one can find such documents only in cities. When cities did not publish Bürgerbücher one may find information about new citizens in their yearly accounting books (Stadtrechnungen or Ratsprotokolle). Often Bürgerbücher were published. A search in KVK is recommended. http://www.ubka.uni-karlsruhe.de/hylib/en/kvk.html
This is a databak which will give titles of Bürgerbücher and where such titles can be found.
Source: GenWiki Bürgerbuch
The Family History Library also has Bürgerbücher which can be searched under Keyword search. Enter: Bürgerbücher