Munster head coach Johann van Graan was left to rue missed opportunities and unfavourable refereeing decisions after seeing his side lose the second of their Champions Cup Pool Two back-to-back matches against Castres Olympique by a single point.

The Top 14 champions held on to win 13-12 at Stade Pierre Fabre, despite playing with 14 players for a total of 20 minutes. Munster failed to score a single point even when they had an extra man at either end of the second period.

"You've got to adapt to the referee on the day," Van Graan said. "There was a lot going on at the breakdown - some decisions went our way and some didn't.

"Regardless of the breakdown, we had enough opportunities to get some more points on the board. We didn't - and well done to Castres for winning by one point in a really tense game of rugby."

He dismissed suggestions, however, that Munster had been unable to create any try-scoring chances in a niggly game in which the sole try came from Castres' Paea Fa'anunu.

"There was Andrew Conway's try-scoring opportunity. I thought Conor Murray created more try-scoring chances that would have made it a game with a bigger margin, and I also thought there was an opportunity after the yellow card at the end of the first half."

Castres earn a penalty after a late scrum

Munster turned down a simple three points after the hooter at the end of the first half, when Rory Kockott saw yellow five metres from his own line, but failed to cross the whitewash.

"We've got to be more clinical," van Graan said. "We certainly created a lot of opportunities tonight, but we weren't good enough to win away - if you can't convert all your opportunities for points, you aren't going to win."

He refused to be drawn when asked about two incidents in the game. "All I can say is the values of rugby are pretty important. I thought we stayed within the values of rugby tonight and hope that due process will be followed.

Castres fullback Scott Spedding said the French champions played much better than they had six days previously in Limerick, when they conceded three second-half tries to lose 30-5.

"The conditions were a lot tougher last week, with the wind and rain. Conditions weren't great tonight, either, but we felt a lot stronger out there," Spedding said.

"Last week we turned at half-time at 6-0, but we never really felt we were in it. We felt we were under pressure all the time, and it would be just a matter of time before the dam wall broke.

"This week we didn't feel like that. At half-time it was close, but we felt we were really in with a shout if we could keep them under pressure."