Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD 192641

We present the results of an optical spectroscopic study of the massive Wolf-Rayet binary HD192641=WR137. These 1986-2000 data cover the dust-formation maximum in 1997. Combining all available measurements of radial velocities, we derive, for the first time, a spectroscopic orbit with period 4766 ± 66 days (13.05 ± 0.18 years). The resulting masses, adopting i=67 ◦ , are MO=20 ± 2M⊙ for the O component and MW R=4.4 ± 1.5M⊙ for the WR component. These appear, respectively, ∼ normal and on the low side for the given spectral types. Analysis of the intense multi-site spectroscopic monitoring in 1999 shows that the CIII λ5696 and CIV λ5802/12 lines have the highest intrinsic variability levels. The periodogram analysis yields a smallamplitude modulation in the absorption troughs of the CIV λ5802/12 and HeI λ5876 lines with a period of 0.83 days, which could be related either to pulsations or largescale rotating structures as seen in the WN4 star EZ Canis Majoris (WR6). Wavelet analysis of the strong emission lines of CIII λ5696 and CIV λ5802/12 enabled us to isolate and follow for several hours small structures (emission subpeaks) associated with density enhancements within the wind of the Wolf-Rayet star. Cross-correlating the variability patterns seen in different lines, we find a weak but significant correlation between the varability in emission lines with different ionization potential, i.e. in lines formed at different distances from the WR stellar core. Adopting a β wind-velocity law, from the motion of individual subpeaks we find β ∼ 5, which is significantly larger than the canonical value β ≃ 1 found in O-star winds.