Shadow catcher in Cycles?

I have modeled an Objekt an set up my lights so that I get some soft shadows and rendered it with Cycles. It looks good so far.
But I have one question: How can I set up the floor to just receive the shadows? I need the transparency around the shadow and my objekt. Using blending modes isn’t an option in this case.

Thanks for your help.

OK thanks. But unfortunately that didn’t help me with my problem. Maybe it isn’t possible at the moment :frowning:

Revision 45242 by Brecht Van Lommel

Cycles: shadow pass support. Note that this only takes into account lamps,
emitting objects or world lighting do not contribute to the shadow pass.

Consider this more as a pass useful for some compositing tricks, unlike
other lighting passes this pass can’t be used to exactly reconstruct the
combined pass.