Junjou Minimum 

A one-volume series that kicks off as a side-story from Junjou Romantica (last chap of volume 4) featuring the characters from Romantica and Egoist. First part flashes back to the younger days of Akihiko Usami and Hiroki while the second part is a cute story about the younger Misaki and his brother... Note: Contains extensive information and an interview with the cast of the drama CD of the same name.