B, V and R band polarimetry of HD 165052

Lomb-Scargle method of period search was applied to the acquired polarization data to identify present periodic signals. To study interstellar polarization in NGC 6530, we observed 25 cluster stars located in close proximity to the binary. Our observations clearly revealed periodic variations of Stokes parameters in all three passbands with the amplitude ~0.1%.

Period search, applied to polarization data, detected unambiguous and strongest periodic signal at 1.47755+/-0.005d, that corresponds to half of the orbital period of 2.95510+/-0.005d. Our independent period search, performed on polarization data, supports a "shorter" value of the orbital period in HD 165052 determined from RV measurements done in the past and rejects "longer" value obtained very recently. In the observed polarization variations, the second terms of the Fourier harmonics are clearly dominating suggesting that light scattering material is symmetrically distributed with respect to the orbital plane.

We concluded that the most probable polarization mechanism is the electron scattering in the interacting stellar winds. To evaluate the effect of non-periodic noise on the orbital parameters derived from analysis of periodic variations of polarization, we have employed rigorous Monte Carlo simulations for the available data. Thus, we derived our best estimate of the orbital inclination i as 55^+5^_-55_{deg}, and {Omega} as 148{deg} (328^+20^_-22_{deg} averaged over B, V, and R passbands.

Our estimates provide direct and unambiguous evidence that non-periodic noise, if present, significantly reduce accuracy of orbital parameters obtained from high-precision polarization data for the low-inclination systems. Using the values of polarization variability amplitude, we estimated the mass-loss rate for the whole system to be (dM/dt=2.46x10^-7^+/-2.47x10^-8^M_{sun}/yr. The direction of the binary system rotation on the plane of the sky is clockwise.

Our observations of HD 165052 neighbour stars revealed complex behaviour of interstellar polarization in the cluster NGC 6530.