Technically, as derivative works, jointly owned by Marc and Roger.. Neither can do anything with them without the other's permission.
My understanding is a little different. Roger's position on 01/11/1996 was as follows:
"MARC MILLER HAS DGP'S PERMISSION to use background data (UWP lists, planet names, sector maps, etc.) from DGP's Traveller works in any way that he sees fit in printed Traveller 4 publications!
In addition, Marc Miller has been given a free hand to dip into DGP's Traveller archives for inspiration in developing background material for his new incarnation of the Traveller role-playing game. This way, he can keep Traveller 4 consistent with what has come before!!!
The races (the Geonee, the Brinn, etc.), the places (Vland, Shudusham, Capital), and the things (robots, equipment, etc.) which you loved from DGP's works still exist in the overall Traveller milieu. I don't believe Marc has any intention of changing this (he's DGP's number one fan!)."
In addition, Marc Miller has been given a free hand to dip into DGP's Traveller archives for inspiration in developing background material for his new incarnation of the Traveller role-playing game. This way, he can keep Traveller 4 consistent with what has come before!!!
The races (the Geonee, the Brinn, etc.), the places (Vland, Shudusham, Capital), and the things (robots, equipment, etc.) which you loved from DGP's works still exist in the overall Traveller milieu. I don't believe Marc has any intention of changing this (he's DGP's number one fan!)."
One point to note: this permission only extends to Marc.
http://members.tip.net.au/~davidjw/tavspecs/maint/dgp_faq.htm
Wil, does this reflect your understanding, or has something changed?