Wales international George North will swap Northampton Saints for Pro14 side the Ospreys, the WRU has announced.
Chris Ashton set a new try-scoring record for a Top 14 season in a 32-17 victory for Toulon over leader Montpellier and Oyonnax beat Brive in the battle of the bottom two on Sunday (AEST).
Toulouse missed the chance to join Montpellier at the Top 14 summit as it fell apart in a 28-23 defeat at Castres.
Chris Ashton equalled the Top 14 try-scoring record as Toulon demolished Clermont Auvergne 49-0 and fellow title contender Racing 92 edged out Lyon.
Top 14 leader Montpellier sits six points clear at the summit after an emphatic display against Castres, Les Cistes winning 45-7.
Former South Africa coach Heyneke Meyer has been appointed as the sporting director and coach of Top 14 struggler Stade Francais.