Watford have signed Didier Ndong on loan from Championship side Sunderland until the end of the season.
And the Hornets have an option to sign the 23-year-old midfielder on a permanent basis in the summer.
Gabon international Ndong has played 54 times for the Black Cats since joining them for £13.6m from French club Lorient in August 2016.
Watford have also signed Sweden Under-21 goalkeeper Pontus Dahlberg on a five-and-a-half-year deal.
Dahlberg, 19, will return to IFK Gothenburg on loan for the rest of this season.
Forward Isaac Success has left the club to join Spanish side Malaga on loan. Another forward, Mauro Zarate, has joined Argentina's Velez Sarsfield on loan.
The club have also released 32-year-old midfielder Ben Watson. Watson - who scored the winning goal in the 2013 FA Cup final for Wigan Athletic against Manchester City - made 73 appearances for the Hornets.
Find all the latest football transfers on our dedicated page.