Eccentricity distribution of wide binaries

A sample of 477 solar-type binaries within 67 pc with projected separations larger than 50 au is studied by a new statistical method. Speed and direction of the relative motion are determined from the short observed arcs or known orbits, and their joint distribution is compared to the numerical simulations. By inverting the observed distribution with the help of simulations, we find that average eccentricity of wide binaries is 0.59 +/- 0.02 and the eccentricity distribution can be modelled as f(e) approximate to 1.2e + 0.4. However, wide binaries containing inner subsystems, i.e. triple or higher order multiples, have significantly smaller eccentricities with the average e = 0.52 +/- 0.05 and the peak at e similar to 0.5. We find that the catalogue of visual orbits is strongly biased against large eccentricities. A marginal evidence of eccentricity increasing with separation (or period) is found for this sample. Comparison with spectroscopic binaries proves the reality of the controversial period-eccentricity relation. The average eccentricity does increase with binary period, being 0.39 for periods from 10(2) to 10(3) d and 0.59 for the binaries studied here (10(5)-10(6) d).