The binary orbit of the Be star HDE 245770 = V725 Tauri and the recurrent X-ray activity of A 0535+26.

Radial velocities of the Be star HDE 245770 determined from 23 spectra obtained at the Observatory of Asiago suggest an orbital period of 55.78±0.64 days. By enlarging the sample with the 27 Victoria radial velocities by Hutchings et al. (1978), an improved period of 55.73±0.31 days and new orbital elements are obtained. According to this orbital solution, recurrent X-ray flares of A 0535+25 should start at phase -0.06±0.03 from periastron and reach the maximum at phase +0.02±0.02. By excluding two giant outbursts, the corresponding figures are 0.00±0.03 and +0.06±0.03 respectively. A much less likely orbital solution with a 111 day period is obtained from either the Asiago or the Asiago and Victoria radial velocity determinations. The equivalent width of the Hβ emission line is also examined.