Both Millie and Walter Karig have two series in common and two pen names: The Nancy Drew Mystery Stories under the pen name of Carolyn Keene, and the Doris Force series under the pen name of Julia K. Duncan. Karig wrote volumes 8-10 of the Nancy Drew series and volumes 3-4 of the Doris Force Series. The Doris Force series consisted of these volumes:

• volume #1 Doris Force at Locked Gates 1931 (Benson)
• volume #2 Doris Force at Cloudy Cove 1931 (Benson)
• volume #4 Doris Force at Raven Rock 1931
• volume #3 Doris Force at Barry Manor 1931

Pictured below are the two Karig volumes:

Doris is an orphan who lives with her uncle. Doris attends boarding school at Barry Manor and gets tangled up in mysteries. These books were originally published by Altemus and then later by Donohue followed by Goldsmith.