Rendering models with shadows on transparent background with V-Ray for Rhino

[how_arkitekter]

Hello, as the title says I would like to create an invisible material that casts a shadow and reflects/refracts the light in the scene. I have modelled a piece of furniture that I've placed in a studio environment in Rhino in order to get cast shadows and reflections, and I want to render this piece of furniture with only the cast shadows and not the background so that I later can place the model in a picture in Photoshop. How can I render the model with cast shadows without the background?

I'm using the latest V-Ray and Rhino.

Thank you