xxx.yyy.distupgrade files in /etc/apt/sources.list - what are these?

I have just used do-release-upgrade to upgrade my system to xubuntu

It looks as this upgrade has created some oddly named files in
/etc/apt/sources.list, these are:-

-rw-r--r-- 1 root root 78 Dec 3 16:12 owncloud.list.distUpgrade
-rw-r--r-- 1 root root 48 Dec 3 16:12 syncthing.list.distUpgrade

Can anyone tell me what these files are for?