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

[Chris Green]

I have just used do-release-upgrade to upgrade my system to xubuntu
17.10.

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?

-- 
Chris Green