The apsidal motion test in 4U 0900-40

Recent SAS 3 observations of 4U 0900-40 have confirmed the apparent eccentricity of the orbit. Arguments are presented to show that the measured eccentricity is not a spurious effect resulting from systematic reflection of the X-ray beam within the binary system. The longitude of periastron is not observed to have advanced in the 3.5 years since its original determination. The limit on apsidal motion is omega not greater than 3.8 deg/yr which, in turn, restricts the apsidal motion constant to log k not greater than -2.5. These limits are used to constrain the physical parameters of the companion star HD 77581.