Paris - Record four-time Celtic League winners Ospreys saw their interest in the European Cup almost certainly ended Friday with a 15-9 defeat at French champions Castres.
With three defeats from three and just one defensive bonus point, Ospreys would need a minor miracle to progress now.
In a dour affair, four penalties from South African scrum-half Rory Kockott and one from his second-half replacement Geoffrey Palis was enough to give Castres their second win in three Pool 1 matches.
But their failure to earn a bonus point in a group featuring Leinster and Northampton, who meet at Franklin's Gardens on Saturday, could come back to haunt them.
Ospreys's nine points came from the boot of Wales fly-half Dan Biggar.
But they didn't play like a side resigned to their fate in the first half, taking the lead through Biggar.
Kockott missed his first two kicks at goal but finally landed the third while Ospreys's Fiji wing Aisea Natoga sat in the sin-bin.
Biggar and Kockott exchanged penalties again before the half-time break and the South African added another at the start of the second period.
Castres knew they might need a bonus for the long run and went in search of tries but Paul Bonnefond spilled the ball five yards out as the hosts turned the screw.
Romain Cabannes was next to blow a try-scoring opportunity with a horrible wayward and forward pass.
But replacement Ospreys flanker Sam Lewis was yellow-carded and Kockott extended the hosts' lead.
Two more penalties, one for each side, left Castres leading by six points with time running out.
But both Palis and Seremaia Bai were sin-binned in the final minute giving the French champions a nervy last few moments holding out on their own line with just 13 men.
Cardiff Blues took charge of the Pool 2 containing reigning champions Toulon, who travel to Exeter on Saturday, with a 29-20 win over Glasgow.
Cardiff scored tries through Rhys Patchell and Alex Cuthbert with Glasgow replying with two of their own for Sean Maitland and Leone Nakarawa but it was Leigh Halfpenny's kicking -- he scored 16 points -- that made the difference.
The Scots were under early pressure, already down by a penalty each for Halfpenny and Patchell, with Cuthbert causing havoc on several rampaging runs.
Pinned down almost on their own line, scrum-half Nikola Matawalu passed back to fly-half Duncan Weir who spilled the ball in his own end zone.
From the resultant scrum, Cuthbert again battered away at the Scottish defence before Patchell wriggled over for the game's opening try.
Halfpenny converted and added another penalty to put Cardiff 16-0 ahead but Glasgow's New Zealand-born Scotland wing Maitland scythed past two tackles just before half-time to bring the visitors back into the match.
Weir's conversion made it 16-7 at the break.
Halfpenny and Weir swapped a pair of penalties each before Cuthbert intercepted a pass deep inside his own half and sprinted almost the length of the field to break Glasgow's resistence.
Nakarawa did go over late on with Weir converting but it was too little, too late.
Cardiff have two wins with each of the other three in the group on one win.