1998A&A...338..947V

We present phase-resolved optical photometry and spectroscopy of the supersoft X-ray binary 1E 0035.4-7230 (=SMC13). The average U, B, V, and R light curves show an almost perfect sinusoidal Δm∼0.3mag modulation on the 4.1hr orbital period, but in individual binary orbits there can be significant deviations from the average light curves. There is a significant B-V variability with a standard deviation of 0.04mag, and which may be related to the orbital phase. The binary appears to be bluest ∼0.2-0.3 in orbital phase before optical minimum. Modeling of the light curves indicates an orbital inclination of ∼20-50° and a mass ratio M₁/M₂<3 (with M₁ the mass of the accreting star). The observed optical modulation is probably dominated by the changing aspect of the irradiated companion star, which has effective temperatures on the irradiated side >30000K. Our spectra show variable HeIIλ4686 emission, and broad Balmer absorption lines. The latter appear more prominent when the companion star is closest to us. Modeling the optical spectrum of the irradiated companion star by a composition of normal stellar spectra reproduces the observed colour and amplitude of the modulation, but predicts too strong Balmer absorption lines which must be significantly weaker because of strong irradiation.