A first detailed study of the colliding wind WR+O binary WR 30a

We present a detailed, extensive investigation of the photometric and spectroscopic behaviour of WR 30a. This star is definitely a binary system with a period around 4.6 d. We propose the value P = 4.619 d (sigma = 0.002 d). The identification of the components as WO4 + O5((f)) indicates a massive evolved binary system; the O5 component is a main-sequence or, more likely, a giant star. The radial velocities of the O star yield a circular orbit with an amplitude K-o = 29.9 (sigma = 2.1) km s(-1) and a mass function of 0.013 (sigma = 0.003) M-.. The spectrum of WR 30a exhibits strong profile variations of the broad emission lines that are phase-locked with the orbital period. We report the detection of the orbital motion of the WO component with K-wo = 189 km s(-1), but this should be confirmed by further observations. If correct, it implies a mass ratio M-wo/M-o = 0.16. The star exhibits sinusoidal light variations of amplitude 0.024 mag peak-to-peak with the minimum of light occurring slightly after the conjunction with the O star in front. On the basis of the phase-locked profile variations of the C iv lambda 4658 blend in the spectrum of the WO, we conclude that a wind-wind collision phenomenon is present in the system. We discuss some possibilities for the geometry of the interaction region.