TIME-SERIES SPECTROPOLARIMETRY OF THE SHORT-PERIOD WOLF-RAYET+O STAR BINARY CQ CEPHEI

We present time-resolved, very precise Linear spectropolarimetry of th e short-period WR+O star binary CQ Cephei. We find that the N IV lambd a 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 systemi c velocity gamma = -72 +/- 1 km s(-1). We identify an antiphase spectr al feature that we attribute to He I lambda 4143 absorption by the O s tar. A semi-amplitude of 360 +/- 18 km s(-1) is found for the star, gi ving a mass ratio (WR:O) of 1.24 +/- 0.06. Spectropolarimetric measure ments, combined with published photopolarimetry, enable a determinatio n of the orbital inclination (i = 82 degrees +/- 0.5 degrees). The emi ssion-line polarization vectors allow us to separate the intrinsic and interstellar polarizations, and we find that the polarization attribu table 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 b inaries are systematically lower than those computed from polarimetric measurements. The light curves from Hiltner an analysed with LIGHT2 t o confirm that the system is in a near-contact state. The resultant ma sses and astrophysical parameters show a normal main-sequence O star w ith a WR star that has parameters typical of the WN 6 type. The distan ce 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 O B1 association by Garmany & Stencel. An evolutionary model involving c ase A mass transfer and stellar-wind mass loss is probably required to explain the present state of this system.