alpha artifact using z combine

hi, i used z combine like in this web page

i have some issues in the alpha foreground image: alpha in the foreground layer is biggest than the backgroud and i see a white edge(MAYBE IS THE SKY COLOR, IF I US DILATE/E ERODE NODE I SEE ALL THE SKY )…how can i fix this artifact…
i read in site that there is an AA mask but i don’t find it

Z-Combine isn’t the most righteous process. I always use “All Z” to cull occluded geometry and then use alpha over to recombine the layers.
The mask you read about is an automatic post-process function over which you have no control.