1. Azzopardi, M. and Vigneau, J. 1975, Astron. Astrophys. Suppl. 22, 285
2. Castor, J.I., Abbott, D.C., Klein, R.I. 1975, Astrophys. J. 195, 157
3. Chiosi, C., Nasi, E., Sreenivasan, S.R. 1978, Astron. Astro phys. 63, 103
4. Falk, H.T. 1979, thesis
5. Fusi-Pecci, F., Renzini, A. 1975, Astron. Astrophys. 39, 413