Time-series spectropolarimetry of the short-period Wolf-Rayet+O star binary CQ Cephei

We present time-resolved, very precise Linear spectropolarimetry of the short-period WR+O star binary CQ Cephei. We find that the N IV lambda 4058 line is the best spectral diagnostic of the WR orbital motion, and we derive a semi-amplitude K-WR = 290 +/- 1 km s(-1) and a systemic velocity gamma = -72 +/- 1 km s(-1). We identify an antiphase spectral feature that we attribute to He I lambda 4143 absorption by the O star. A semi-amplitude of 360 +/- 18 km s(-1) is found for the star, giving a mass ratio (WR:O) of 1.24 +/- 0.06.

Spectropolarimetric measurements, combined with published photopolarimetry, enable a determination of the orbital inclination (i = 82 degrees +/- 0.5 degrees). The emission-line polarization vectors allow us to separate the intrinsic and interstellar polarizations, and we find that the polarization attributable to scattering of the WR radiation in the expanding wind is small (< 0.3 per cent), but may be increasing towards the UV. We note that the inclinations derived from photometric analyses of eclipsing WR+O binaries are systematically lower than those computed from polarimetric measurements.

The light curves from Hiltner an analysed with LIGHT2 to confirm that the system is in a near-contact state. The resultant masses and astrophysical parameters show a normal main-sequence O star with a WR star that has parameters typical of the WN 6 type. The distance modulus for CQ Cep derived from our results is (V-0 - M-V) = 12.0, in excellent agreement with the value of 12.2 determined for the Cep OB1 association by Garmany & Stencel. An evolutionary model involving case A mass transfer and stellar-wind mass loss is probably required to explain the present state of this system.