They Were 11!

Ten elite space cadets, from different races spread across the galaxy, are assigned to board a decommissioned spaceship as their final test. Their orders are to survive as long as they can with what they have. Once in the ship, they find that their crew has gained an eleventh member. Given the circumstances of the cadet selection process and how rushed the exams were, no one can remember the original members well enough to recognize which is the new face nor how they got there.
As the days pass, the eleven must deal with their suspicions of each other as well as the sudden news that the spaceship is in dire straits.
Winner of the 1976 Shogakukan Manga Award, They Were Eleven is heralded as one of the turning points in shojo manga history for its prespectives on the sci-fi genre and gender roles.