The massive double - lined O - type binary HD 165052

We present a new optical spectroscopic study of the O-type binary HD 165052 based on highand intermediate-resolution CCD observations. We re-investigated the spectral classification of the binary components, obtaining spectral types of O6.5V and O7.5V for the primary and secondary, respectively, finding that both stars display weak C iii λ5696 emission in their spectra. We also determined a radial-velocity orbit for HD 165052 with a period of 2.95510 ± 0.00001 d, and semiamplitudes of 94.8 and 104.7 ± 0.5 km s, resulting in a mass ratio Q = 0.9. From a comparison with previous radial-velocity determinations, we found evidence of apsidal motion in the system. Several signatures of wind-wind collision, such as phase-locked variability of the Xray flux and the Struve-Sahade effect, are also considered. It was also found that the reddening in the region should be normal, in contrast with previous determinations.