There was a mistake in my last post!
Correct is: “A friend of ours lives in Kassel.”
Not right: “A friend of us lives in Kassel.”
To describe relationships between people, we either
1) put the possessive determiner before the other person or
2) put the possessive pronoun at the end.
Both are correct: 1) They are my friends and 2) They are friends of mine.