Synoptische Überlieferung bei den apostolischen Vätern, 1957 title page (Helmut Köster)
Introduction to the New Testament, 1982 title page (Helmut Koester)
Harvard Divinity School website, viewed August 16, 2019 (In memoriam Helmut Koester; Helmut Koester, John H. Morison Research Professor of Divinity and Winn Research Professor of Ecclesiastical History; died January 1, 2016; came to Harvard after teaching at the University of Heidelberg from 1956 to 1959) https://hds.harvard.edu/alumni-friends/2016-deans-report/memoriam-helmut-koester
Wikipedia, viewed August 16, 2019 (Helmut Koester; full name: Helmut Heinrich Koester; born December 18, 1926, in Hamburg; died January 1, 2016; German-born American scholar of the New Testament and early Christianity at Harvard Divinity School; research areas: New Testament exegesis; history of early Christianity; and archaeology of the early Christian period; began teaching at Harvard in 1958)