Occlusion with invisible objects

I've been rendering a effects test in multiple layers for a few days now, and I'm on the last step, the lens flares. Now, I want to have the flare I'm using for my star to occlude whenever one of the scene's objects passes over it, but I also want the objects to be invisible. I've played with the options on the properties rollout, but turning the objects invisible just negates the occlusion effect.

So, how would I go about getting both the invisibility and the occlusion to work?

I'm using Max 4.

 

Make them black.

 

Rendering still takes a while, but it does work. Thanks.

 

Or use a matte/shadow material on them... that way you can get a correct alpha-channel...