The binary orbit of the Be star HDE 245770 and occurrence of outbursts in A0535+26 after periastron passage

Radial velocities of the Be star HDE 245770 determined from 23 Asiago spectra indicate an orbital period 55.78±0.64 days. The orbit satisfies at 1 σ level all four constraints imposed by Nagase et al. (1982) for the optical counterpart of the X-ray recurrent transient A 0535+26. By enlarging the sample with 27 radial velocities by Hutchings et al. (1978), the improved period (55^d.72±0.31) and elements are obtained.