These are things that are usually found based on their personality, name, or research done indept on the characters. While not everybody may consider these hints, we do.
Total Proof: 1
1. They're name compatible. Before you go all "O__o", I'll explain. Hoshikawa Subaru more or less means, "Earth and Stars". Shirogane Luna means "Platinum Moon". What do these names have in common? They all have to do with space. For those who go "What about Misora? Her name means beautiful sky!" Yes, it does. Sky is a part of Earth, but not space, which is the common factor in Subaru's full name, not to mention that space and astrologoy is Subaru's passion. Enough said.