Castres bounced back from last week's defeat to Leinster by finishing up hugely impressive 41-7 victors against Northampton Saints.
The French side registered five tries, including a fine brace from winger David Smith, and are now just a point behind Leo Cullen's side, who sit top of Pool 4.
Former Leinster out-half Jimmy Gopperth sent over a conversion in the dying moments to earn Wasps a 20-20 draw with Toulouse.
The late try was scored by Nathan Hughes as Wasps clawed their way back at the Stade Ernest Wallon, but they remain two points off Pool 2 leaders Connacht.
Another ex-Leinster playmaker, Ian Madigan, started in the out-half berth for Bordeaux-Begles, but he was unable to prevent them losing 49-33 to Clermont Auvergne.
Madigan converted two of Bordeaux's four tries, but Clermont ran in six of their own as French scrum-half Morgan Parra contributed 17 points from the tee.
The bonus-point win leaves Clermont five points clear at the summit of Pool 5. Despite their win over Exeter, Ulster are in third, a point behind Bordeaux.
Freddie Burns racked up three second-half penalties, and converted his own breakaway try, as Leicester defeated Ronan O'Gara's Racing 92 27-17 at Welford Road.
The Tigers are now a point behind Glasgow Warriors and Munster who, after their outstanding victory at Thomond Park, top Pool 1.