The Be/X-ray binary LS I +61 235/RX J0146.9+6121: physical parameters and V/R variability

We present the results of spectroscopic and photometric observations of LS I +61 235. Blue 4100-4900Å spectra of the system are used to assign a spectral type of B1V to the optical companion while detailed uvbyβ photometric data permit us to derive its astrophysical parameters. The monitoring of this high mass X-ray binary since its discovery in 1991 has resulted in the detection of considerable variability in the Hα line profile. The observations cover a complete V/R cycle with a period of approximately three years. We discuss models which may explain the origin of the variation and conclude that these variations are due to intrinsic changes of the circumstellar envelope rather than being caused by the neutron star. The model which best fits the observational data is the global one-armed oscillation model.