Using a Mask from the Motion Tracker in Alpha-Rendering?

Hello guys,
I´m having a bit of a problem here and would be glad about any clues. I set up a scene in the motion tracker to add some visual effects to it. Then I masked a foreground object in the image sequence. Now I need to do an additional rendering with only the effect with alpha for external compositing.
I just can´t get the mask get rendered as alpha aswell. The mask shows up black in the rendered image (not the desired checker-pattern).
Is there a way to have the mask rendered with alpha?

Thank you for your help!

You can extract the foreground object with alpha by exporting as PNG with the RGBA selected in the render ouput settings. I’m guessing you had it set to RGB without the A.

Steve S

Thanks Steve, but rendering the foreground with alpha is actually not the problem. I need the black area (my mask) in my screenshot to be rendered as alpha as well.