Make a mesh invisible but still cast shadows

Hey Unity-Fans :)

My weekly buglist only shows one last unfixed bug in my prototype and it doesn't seem that i can fix it myself, so i ask you guys.

How do i make a mesh invisible but still cast shadows... To be a little bit more specific:

I have a scene with my character and a mirror. i want my character to be invisible for my camera but not invisible for my mirror. my camera should only see the shadows without the mirror.

Does anyone got a clue or a nudge in the right direction? That would be awesome :)