Final render too transparent when using transparent background

I am creating a shooting star using the smoke simulation, and rendering it in Cycles. In the preview render, the smoke and fire are opaque and look how I want them too. With out changing any settings I rendered the final render, but the smoke and fire were very transparent!



Any help will be much appreciated.
Here is the blend file: http://www.pasteall.org/blend/33507

I have run into the same problem when trying to render particles on a transparent background using DOF. In the preview render, it looks perfect, but in the final render the particles are extremely transparent.



-In the preview render it works correctly with DOF and transparent background
-In final render it is too transparent with DOF and transparent background
-In final render it works correctly with an opaque background
-In final render it works correctly with transparent background without DOF

I need to render an image with a transparent background AND depth of field, but this problem is prohibiting me from doing it. I would really appreciate any help.

Here is the blend file of the scene shown above: http://www.pasteall.org/blend/33517