Object render with hdri

[irmin]

I'm trying to render an object with an HDRI background (+ illumination), without a surface for the object to be placed on, so that it seems to rest on the 'ground' of the HDRI picture. Ofcours this way there are no shadows on the ground. I tried with a transparant surface underneeth the object, but no go... is there an quick and easy way to go about this, directly in vray or SU, without having to turn to PS afterwards?

thx,
Irmin