Being an irreligious person I don’t visit churches much – but the older they get and the closer to some pantheistic, or pagan root – the more interesting they become. So to happen upon this rather striking conjunction of I3th. C. building – La Chapelle de Notre-Dame de Centeilles – and these Pierres Dressées, or Plantées as I first took them to be, was a real delight. The chapel itself was closed, but we returned on sunday when it is open to the public for two hours, and staffed by members of Les Amis de Centeilles Association. It was then I was told that the two stones were the vestiges of a dolmen à couloir or passage-grave, dated to the Bronze or possibly Chalcolithic Age.
The orthostats are aligned N – S, which in this region isn’t so unusual as the various influences from Brittany, from the Alps, from the Mediterranean isles and from Iberia all left their imprint on funerary architecture and practices. Thus we can find alignments almost all round the compass – though mainly SE, or SW facing.
Above is the left or southern stone. It measures 3m. by 1m.
The righthand stone is 2.5 m. long by 2m. tall. They are both 30 cm. thick. The dry-stone infill resembles the design of the Mourel des Fados dolmen – the longest passage-grave in the south of France at 25m. – which lies not 10 km from here.
Dolmens Lost and Found 
This stone alignment is very strange and I believe that you are right, they are the vestiges of a dolmen. It was large and certainly that some of its flagstones were used for the construction of the church or other buildings of the surroundings.