Leinster have past history with all three of their Pool 2 opponents in the European Rugby Champions Cup. Indeed, they lined out against two of them - London Wasps and Castres Olympique - in the last 13 months.
EUROPEAN RUGBY CHAMPIONS CUP - POOL 2:
Today's draw in Neuchatel, Switzerland saw top seeds Leinster placed in Pool 2 where they will battle it out with Castres and English sides Harlequins and Wasps.
The province beat Castres home and away in the 2013/14 Heineken Cup, their away win breaking the French club's year-long unbeaten home record.
Leinster last faced Harlequins in European competition on the way to winning the Heineken Cup in 2009. Back then they won a tight game at the Stoop by a single point to set up a Croke Park semi-final showdown with Munster.
Meanwhile, in April of last year, the Blues got the better of Wasps in a free-scoring Amlin Challenge Cup quarter-final. Five tries from Ian Madigan, Mike Ross, Isa Nacewa, Rob Kearney and Gordon D'Arcy all contributed to a 48-28 victory.
Giving his reaction to the Champions Cup draw, hooker Sean Cronin said: "Usually there's only one group of death, but they all look tough pools. We've got two trips to London which all the boys will be looking forward to.
"Hopefully there are a few Irish living in London who fancy coming along and giving us their support, as they did at Northampton last season.
"We had Castres in that pool as well. We played poorly at home and just squeezed out the win but did well out there when we knew it was do-or-die for qualifying.
"But they have since reached a second Top14 final in a row so that says everything about their strength. They may not be the first French team people talk about, but you don't reach two finals in a row and win the title without being a top-quality team."
Reflecting on Leinster's exit from the Heineken Cup at the quarter-final stage, he added: "We were disappointed with ourselves this season. We work so hard to progress from a pool that including Northampton, Castres and Ospreys and then felt we let ourselves down in the quarter-final at Toulon.
"Leinster have a proud tradition in Europe and it will be a huge focus for us next season. We are judged on winning trophies and it was great to lift the PRO12 title and send everyone off for the summer break with a huge boost but we want to prove ourselves again in Europe next season, even though it will be strange not to have Leo (Cullen) and Brian (O'Driscoll) on the pitch."
Dave Kilcoyne scored the crucial try two minutes from time - and captain Tyler Bleyendaal added the conversion - as Munster staged an unlikely comeback victory away to the Ospreys to stay top of the GUINNESS PRO12.
Ulster put in one of their finest displays of the season at Kingspan Stadium, running five tries past fellow top four hopefuls Glasgow Warriors to leapfrog the Scots into fifth place in the GUINNESS PRO12 table.