Weiss, A. Stellar Astrophysics, MPI for Astrophysics, Max Planck Society;
Troisi, F., Bono, G., Stetson, P. B., Pietrinferni, A., Weiss, A., Fabrizio, M., et al. (2011). On a new parameter to estimate the Helium content in old stellar systems. Publications of the Astronomical Society of the Pacific, 123(906), 879-891. doi:10.1086/661534.