Le dernier jour

I recently saw this movie on the International Filmfestival in Rotterdam and was wondering if someone knew who did the soundtrack. I have this song humming in my head, but cannot find the title and performing artist(s)... The movie is about a young French boy who's being followed on the train from Paris to his home on the coast by a girl. So he brings this girl home with him and his parents assume they are boy and girlfriend. When the girl is introduced to a friend of the boy she starts to fancy him more and the boy starts to feel lonely. It looks like she is using him. She sleeps with him, but has sex with his mate. When you think you have figured it out, there comes a nice twist in the story! I liked it and of course the music!