A new question mark hangs above the second Toro Rosso cockpit for 2019.
Daniil Kvyat has been signed up already, but it is believed Brendon Hartley has been told he is definitely vacating the other seat.
Robert Kubica was earlier mentioned as one potential driver, but the Pole said last week: "I will definitely not go to Toro Rosso."
Another favourite was thought to be Dan Ticktum, the 2019 Macau GP winner. But the Briton does not have enough points to secure a F1 super license.
And the Japanese series Super Formula has announced on its Facebook page that the 19-year-old "will thrill Super Formula fans next season when he drives full time in the series".
Another big favourite is Alexander Albon, whose Thai links were said to be a potential funding coup for the junior Red Bull team.
But Red Bull was working to extract the 22-year-old from his Nissan contract in Formula E, and those efforts may have failed.
That is because Formula E has just announced its official entry list for 2019, outlining that Albon will indeed race for Nissan.
An outside shot for the Toro Rosso seat could be Naoki Yamamoto, a 30-year-old veteran of the Japanese racing scene with strong links to engine partner Honda.
Yamamoto has just wrapped up the Super Formula and Super GT titles, the latter together with Jenson Button.