Ireland's most capped player Ronan O'Gara has retired to take up a role coaching former rival Jonathan Sexton at Racing Metro.
Castres outplayed Montpellier to make the semi-finals of the Top 14 with a 25-12 home victory.
South Africa fly-half Morne Steyn is to join Stade Francais on a three-year deal, the French Top 14 club said on Tuesday.
Clermont Auvergne notched up a record 60th win in succession at home when defeating Bordeaux-Begles 67-3.
Wallaby winger Digby Ioane has signed a two-year deal to play for Stade Francais in the French Top 14.
Australia winger Digby Ioane is to leave the Queensland Reds and move to a French club at the end of the Super Rugby season.
South Africa winger Bryan Habana is to join French club Toulon for the next two seasons, the Top 14 side said on Tuesday.
Fly-half Francois Trinh-Duc has returned to training for Montpellier after recovering from a sprained ankle.
Drew Mitchell has signed a two-year deal with Toulon starting next season, the French Top 14 club said on Wednesday.
France wing Vincent Clerc has been ruled out for the season with a knee injury and will miss the tour to New Zealand in June.
Clermont have confirmed the signings of scrum-half Thierry Lacrampe and full-back Mike Delany for next season.
Stade Francais beat Paris rivals Racing Metro 19-16 in their Top 14 derby at the Stade de France.
Round-up: Clermont beat Toulouse 39-17 at the Stade Marcel Michelin to secure a home semi-final in the Top 14 play-offs.
Former England fly-half Jonny Wilkinson has signed a new one-year contract with French club Toulon.
Round-up: Toulon retained their place at the summit of the French Top 14 after sharing a 26-26 draw with Clermont on Sunday.
Round-up: Montpellier boosted their Top 14 play-off hopes with a 10-8 win over Toulouse at the Stade Yves du Manoir.