Phase variability in the optical polarization of GRO J 1655 – 40

We present the results of the new optical polarimetric observations of the superluminal source GRO J1655–40, carried out in July 1997, with the multi-channel photopolarimeter of the Torino Observatory, using the 2.15-m telescope of Complejo Astronomico El Leoncito (Argentina). The observed amount of polarization shows (mainly in the V and I bands) oscillations which are consistent with the orbital period of the system. This gives information about physical and geometrical properties of the binary system and confirms that the optical continuum is intrinsically polarized. The origin of the polarized flux is likely to be related to electron scattering of photons from the accretion disk.