The horizontal branch morphology of M 31 globular clusters - Extreme second parameter effect in outer halo clustersS. Perina, M. Bellazzini, A. Buzzoni, C. Cacciari, L. Federici, F. Fusi Pecci and S. GalletiA&A, 546 (2012) A31DOI: https://doi.org/10.1051/0004-6361/201220037
