Our photometric catalogues, including both the large field catalogue obtained by scanning a photographic plate and the small CCD field catalogue, will be published in a companion paper (Slezak et al. 1997). All these optical data are used to give an interpretation of the properties of ABCG85 (Durret et al. 1998). They will be compared to the results already obtained from X-ray data in Papers I and II (Pislar et al. 1997; Lima-Neto et al. 1997).
We are very grateful to Andrée Fernandez for her help during the preparation of the first observing run, to Paul Stein for helping us obtain a catalogue of guiding stars and to Cláudia Mendes de Oliveira for her cheerful and highly competent assistance at the telescope. We thank the anonymous referee for many useful comments. CL is fully supported by the BD/2772/93RM grant attributed by JNICT, Portugal.