Hunch. Thomas Aeschi is one of eight Swiss People's Party candidates named by 23 October, and parliament can elect only one of them on 9 December.
The Aeschi market resolves on the Swiss parliament's page for this election: YES if the first person parliament officially declares elected to Guy Parmelin's seat is Thomas Aeschi, whether or not he accepts. If the vote moves, the market follows it until 30 June 2027, and RFI and swissinfo are the backups.
We looked for cases where a minister from the Social Democrats on the left or the Swiss People's Party on the right left the government in the middle of a term, and parliament filled only that one seat that day. We found five such elections since the seats were first shared out in 1959. Each time the party of the minister who left kept the seat. (Swiss Parliament; Swiss Parliament; swissinfo; RFI)