The Ap star 108 Aqr. I.

Summary. Our photometric and spectroscopic observations of the ApSi 4200 star 108 Aqr confirm the periodic variations with a period of 3.73 days found by Morrisson and Wolff (1971). The spectral type of this star is determined as B5V or B6V. The effective temperature is 14000 0K (Oe =0.36) and the surface gravity, constant throughout the periodic variations, is logg =3.5, which corresponds to the gravity of a dwarf for the method used. We show that effective temperature's variations cannot account for those of colour index. Profile variations for the Ti ii, Sr ii, Fe ii, Cr ii lines can be seen on the microphotometer tracings. We find the mean radial velocity to be 19.8 km 5 1 The Si ii do not show velocity variations, while the radial velocities of the Fe ii and Cr II lines vary slightly and those of the Ti ii lines clearly show variations. Also we can measure the radial velocities of the red and violet components of the Ti ii lines when they show signs of doubling. It is shown that a binary system cannot account for the observed variations (light, line profiles and radial velocity). Key words: Ap Si 4200 star - light, line profiles and radial velocity variations