A new population of extended , luminous globular clusters has recently been discovered in the outskirts of M31 .
These objects have luminosities typical of classical globular clusters , but much larger half-light radii .
We report the first results from deep ACS imaging of four such clusters , one of which is a newly-discovered example lying at a projected distance of \sim 60 kpc from M31 .
Our F606W , F814W colour-magnitude diagrams extend \sim 3 magnitudes below the horizontal branch level , and clearly demonstrate , for the first time , that all four clusters are composed of \gtrsim 10 Gyr old , metal-poor stellar populations .
No evidence for multiple populations is observed .
From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 \lesssim [ Fe / H ] \lesssim - 1.8 .
The observed horizontal branch morphologies show a clear second parameter effect between the clusters .
Preliminary radial luminosity profiles suggest integrated magnitudes in the range -6.6 \lesssim M _ { V } \lesssim - 7.7 , near the median value of the globular cluster luminosity function .
Our results confirm that these four objects are bona fide old , metal-poor globular clusters , albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond .