Volumes 1-5 contain another series titled "Haru-iro Astronaut" (Spring-Colored Astronaut).
Volume 6 contains the side stories "Mirai" (未来) and "Suwa Hiroto" (須和弘人).
Haru-iro Astronaut (春色アストロノート); #1: Bessatsu Margaret 2011-06, #2: Betsuma sister 2012-01, #3~5: kakioroshi
Mami and Tomoki have always shared everything in life, they are identical twins, yet so different in character and personality: Mami tender and helpless, courted by boys to whom she doesn't never "no" for excessive kindness, Tomoki with a strong sense of duty and a great spirit of sacrifice, would do anything to see his sister happy. But comes a time, however, when one realizes there are things cannot be shared, especially when it is something extremely rare and unique... just like an alien?! (Source: MU)
Orange: Suwa Hiroto
A side story focusing on Naho and Suwa's lives in the world where Kakeru passed away at age 17.