O'Brien, who had studied German in Dublin, may have spent at least parts of 1933 and 1934 in Germany, namely in Cologne and Bonn, although details are uncertain and contested. He claimed himself, in 1965, that he "spent many months in the Rhineland and at Bonn drifting away from the strict pursuit of study." So far, no external evidence has turned up that would back up this sojourn (or an also anecdotal short-term marriage to one 'Clara Ungerland' from Cologne). In their biography, Costello and van de Kamp, discussing the inconclusive evidence, state that "...it must remain a mystery, in the absence of documented evidence an area of mere speculation, representing in a way the other mysteries of the life of Brian O'Nolan that still defy the researcher."