[Agemegos]

Originally Posted by Anthony

I don't think white dwarfs can be supernova remnants.

Yes, that's a mistake on my part.

Also, note that the red giant will have affected planets orbiting other stars in the group if they're close by (the red giant was likely thousands of times brighter than the other stars, so if the planetary orbit was is more than a couple percent of the distance to the red giant, the red giant will have been providing more heat than the main star).

Quite right. I meant to imply that, and should have been more clear.

The ideal thing to do would be to calculate the maximum luminosity and radius of the star at its red giant maximum, remove everything that is inside it, generate the whole system by calculating all the planets' and moons' black-body temperatures from combined insolation at their closest approach to the red giant, add the effect of the immense solar winds during the formation of the planetary nebula. Then alter the orbits, calculate the luminosity of the white dwarf, calculate the new values of insolation and black-body temperature, adjust the atmosphere types and masses to account for anything that condenses or freezes out, and calculate the new surface temperature.

Unfortunately GURPS Space doesn't provide most of the tools for that. And even if you figure out how to do it yourself the results are pretty crude.