THE W-URSAE-MAJORIS SYSTEM ER-ORI - A MULTIPLE STAR

308 photoelectric and 29 spectroscopic observations of the W-type W UM a 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 fu nction. High-resolution speckle masking observations show that this co mpanion has a separation of 0.19'' from the W UMa system, is about 2.3 (m) fainter than the binary at maximum light, and is very likely physi cally connected to the system. The (O - C)-diagram of minimum times wa s re-analyzed. The light time effect caused by the third body has only a marginal influence, indicating that its orbital plane has an inclin ation close to 0 degrees. 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 derive d masses and radii indicate that ER Ori is a slightly evolved W UMa sy stem.