Anomalous periastron positions in x-ray binaries

The values of the periastron position (ω) are expected to be uniformly distributed between 0 and 2π if they are true orbital parameters. Instead, all the values that have been determined from optical velocity curves fall within five degrees of ω=0 for X-ray binaries with early type primaries. The observed clustering near ω=0 is explained in this paper by phase-dependent absorption in a stellar wind. A spurious eccentricity is produced by this absorption. However, the peak-to-peak velocity amplitude, which is used in the determination of the neutron star mass, is unaffected to first order in the eccentricity. (author)