Northampton battled hard for an 18-12 Heineken Cup victory over Castres that keeps them in the hunt for quarter-final qualification.
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Ulster were confirmed as Pool Four winners following their victory over Glasgow, but Northampton could still reach the knockout stages as one of the best group stage runners-up.
In bitterly cold temperatures and through thickening fog at Franklin's Gardens, Northampton ground out the victory they needed with three penalties each from Stephen Myler and Ryan Lamb.
The Saints will now go in search of a bonus-point victory over Glasgow next weekend and hope that is enough to secure a berth in the quarter-finals.
That scenario seemed unlikely after Northampton had been thumped at home by Ulster but Jim Mallinder's Saints revived their fortunes with a return victory at Ravenhill.
England coaches Graham Rowntree, Andy Farrell and Mike Catt were in the stands and they will have been impressed by the all-action performance of lock Courtney Lawes, who was named man of the match.
Castres started like a train with the hard-running centre Romain Cabannes crashing through Myler before off-loading to Max Evans, but they failed to utilise Marcel Garvey on the overlap.
Northampton escaped again when scrum-half Rory Kockott missed his first penalty shot at goal after he had been tackled off the ball by Jamie Elliot.
Having already stolen one lineout, Lawes then claimed a towering restart and galloped clear down the wing after Samu Manoa's half-break and offload.
Northampton's counter-attack lost its momentum when Tom Wood dropped a pass behind him and the England flanker was then penalised for side entry at a ruck.
Castres cleared and Northampton were back on the defensive after a powerful charge from the French side's number eight Antoine Claassen.
But Northampton were saved by Lawes, who closed down Cabannes and executed a thunderous tackle which stopped Castres from exploiting another Garvey overlap.
Kockott kicked Castres into the lead after referee George Clancy penalised the Northampton scrum, despite the Saints pack being on top and going forward.
Referee Clancy evened the ledger at the very next scrum, allowing Myler to equalise but his handling of the set-piece raised more than a few eyebrows in the stands.
Ben Foden made a half-break to keep the crowd warm in bitterly cold conditions before Soane Tonga'uiha charged for the line. He was stopped short and Northampton awarded the scrum.
The Saints pack was penalised for wheeling the scrum and Castres cleared but Northampton had better luck at the next set-piece, winning a penalty which Myler kicked successfully.
Castres equalised immediately, though, when Northampton were penalised for sealing off the ball under their own posts and the first half ended with the scores level.
But Northampton should have been leading. Their driving maul was proving a valuable attacking weapon and Anton Peikrishvili was sin-binned for pulling it down as the Saints rumbled towards the line.
Northampton hooker Dylan Hartley spurned the points and went for another catch-and-drive against the seven-man Castres pack but the Saints made a hash of it, ended up going backwards and lost the opportunity.
Castres fly-half Daniel Kirkpatrick - a distant relative of the All Blacks legend Ian Kirkpatrick - hit the post with a drop-goal attempt early in the second period.
Calum Clark, named in England's Six Nations squad this week, charged down Kockott's box kick and was then held back by the Castres scrum-half.
But Phil Dowson launched the counter-attack, Myler made a half-break and then off-loaded to Hartley, who came within inches of scoring the opening try but was held up.
Northampton won the put-in and attacked the blindside, with Foden arrowing for the corner. He was halted short but Dowson was bundled into touch.
Dominic Waldouck tried to conjure an opening but was swallowed up by the Castres defence and eventually Northampton had to settle for a third Myler penalty.
Northampton then repelled Castres, holding firm in the scrum before some aggressive defence forced Joe Tekori to hold on and concede the penalty, allowing Myler to clear.
Myler was replaced by Lamb who extended Northampton's lead, but it was cancelled out by Kockott almost immediately after the Saints' replacement fly-half had been penalised for going off his feet.
Lamb then almost had a pass intercepted but the tension was eased a shade when he chipped over another penalty, after Yannick Forestier had been penalised at the ruck.
Lamb then sealed the victory with his third penalty, although Kockott rescued a losing bonus point for Castres with the final kick of the game.