1994A&A...289..827G

308 photoelectric and 29 spectroscopic observations of the W-type W UMa system ER Ori are presented and analyzed. A cross-correlation method was used to determine the radial velocity curves. The existence of a third body was established by its presence in the cross-correlation function. High-resolution speckle masking observations show that this companion has a separation of 0.19" from the W UMa system, is about 2.3mag fainter than the binary at maximum light, and is very likely physically connected to the system. The (O-C)-diagram of minimum times was re-analyzed. The light time effect caused by the third body has only a marginal influence, indicating that its orbital plane has an inclination close to 0deg. The presence of the companion was taken into account in the synthetic light and radial velocity curve study. This combined solution yields a mass ratio of q=0.64 ±0.08. The derived masses and radii indicate that ER Ori is a slightly evolved W UMa system.