Vermehren converted to Roman Catholicism in 1939, shortly after his elder sister Isa,[citation needed] when he met the Countess Elisabeth von Plettenberg, a member of one of Germany's leading Catholic families.
At that time, Admiral Wilhelm Canaris, head of the Abwehr, was making peace overtures to the Americans with the help of the German ambassador to Turkey, Franz von Papen, who had been asked to meet Archbishop (later Cardinal) Francis Spellman.
[citation needed] In Sofia, the ambassador was a family friend who (in cooperation with the Abwehr station chief) sneaked her on board a diplomatic courier plane to Istanbul.
[citation needed] The couple received word that a friend from the Foreign Office, Otto Carl Kiep, had been arrested on 12 January 1944 in connection with his attendance at the Frau Solf Tea Party.
[citation needed] British propaganda organisations broke the news of the defection, knowing that it would cause havoc in Germany's intelligence services, especially since the invasion of Western Europe was imminent.
Isa wrote of her time in prison camps in the book, A Journey through the Final Act: Ravensbrück, Buchenwald, Dachau: A Woman Reports.
[4] The Vermehrens were given accommodation in the South Kensington flat of the mother of Kim Philby, where they provided him with lists of personalities of the Catholic underground in Germany.
[citation needed] They settled near the Church of the Immaculate Heart of Mary in Brompton Road, and changed their surname to Vermehren de Saventhem for genealogical reasons.