Leinster will be the first in action as they prepare to host Castres in their Pool 4 opener at the RDS.
The Blues are without the influential Johnny Sexton for this afternoon's game after the Ireland out-half complained of a tight hamstring in training this week.
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Twenty year-old Joey Carbery will start at no.10 today.
Former Leinster captain Brian O'Driscoll says it's a big opportunity for the young out-half.
Kick-off at the RDS is at quarter-past three.
Meanwhile, there's a half-past five start this evening at the Sportsground where Pro12 champions Connacht will begin their Pool 2 campaign when they face French giants Toulouse in Galway.
