Ulster now have to wait to see how Toulouse, Saracens and Montpellier fare in their final pool games to find out if they will be home or away in the knockout stages of Europe's top tournament.

A bonus-point victory would have secured a home draw but Ulster, who lost last season's Heineken Cup final to Leinster at Twickenham, never looked like scoring four tries in a tense encounter at the Stade Pierre Antoine.

Talisman and inspiration Pienaar was switched from scrum-half to fly-half for the trip to France, with first-choice Paddy Jackson ruled out with an ankle injury and Paul Marshall at scrum-half.

Castres opened the scoring when Scotland wing Max Evans' superb behind the back pass gave his centre Remi Lamerat a straight run-in from halfway to give the home side a ninth-minute lead.

Ulster were in control at the scrum and put the home side's pack under pressure throughout the contest and not even Castres' former Ulster No8 Pedrie Wannenburg had an answer to their power in that area.

Pienaar and his fellow South African, Rory Kockott, swapped penalties in a tight first half to make it 8-6 at the break and the Ulster half-back's solitary second-half penalty after 63 minutes secured the win.