When I did it I took the Voie Laurent Fignon........ and I've copied a brief description
"On entering Barèges the road kicks again, reminding you once more that the Tourmalet means business but it’s at mid distance when choices have to be made. Remain on the D918 and a beautiful set of switchbacks await, however if you’re feeling a little more adventurous then it’s time to head right and onto the Voie Laurent Fignon, the old Tourmalet road that is now only open to cyclists. With faded road markings from the Tour de France this road has a certain energy that can still be felt from the race and, in my opinion, it’s the only road to take on the way up gifting you magnificent views of the main road below and the distant valley.
As you join the main road once again you now have just 4 kilometres to go."
Well worth doing. It was absolutely beautiful and, as the article says..... very atmospheric.