How to maintain a transparent background

I’ve an object that needs an outline at the end of the animation and a transparent background. I’ve come up with this but then the backgroudn gets opaque…

Thanks for the help, Twan

There’s no alpha value going into your final composite node, see if that helps

but it is in the renderlayers, why did I loose it and how to get it back then?

@Richard Marklew Thanks I now know what you mean. It works!