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Published online by Cambridge University Press: 23 September 2016
The principal aim is to report on the sudden appearance and disappearance of (mainly) Fe II emission lines in the Be/X-ray system HDE 24-5770/ A0535+26, about 24 days before an X-ray outburst was observed. The system is characterized as an 09-7 Ille star of mass 20-25 M⊙, accompanied by a neutron star (X-ray pulse period ≃104 s), with orbital period -110 d, eccentricity > 0.3; for a more complete description see Giovannelli et al. (1985) or de Loore et al. (1984). X-ray outbursts have been observed recurrently, obviously connected with periastron passage of the neutron star triggering gas infall onto it. The observed X-ray outbursts are of varied strength, from barely detectabl to 3 Crab units at peak (Nagase et al. (1982), Giovannelli et al. (1984)). A weak X-ray event took place on December 13, 1981.