Double Seven (ダブルセブン) is a part of ( 二年度, lit. Second year ) refers to the short segments written by Satou Tsutomu released in Dengeki Bunko MAGAZINE in a timeline when Tatsuya and Miyuki reached their Second year in First High School. It's been published from November 2012 to August 2013.
New characters are introduced: Mayumi's younger twin sisters who enrolled as freshmen, Izumi and Kasumi; Shiba siblings' live-in maid, Sakurai Minami; and a member of the 18 Assistant Houses, Shippou Takuma.
It is now published in light novel format as Volume 12.