1995A&A...297..127H

Absolute dimensions for the detached O-type eclipsing binary Y Cygni are derived, based on analyses of new Reticon spectra and existing photometric data. For the primary (larger) and secondary (smaller) stars respectively, we adopt the following dimensions: m/m_☉=17.5±0.4 and 17.3±0.3, R/R_☉=6.0±0.3 and 5.7±0.3, and logL/L_☉=4.47±0.12 and 4.45±0.12. Both components are shown to have equal temperatures of 31000±2000K although we recognize that there remains a problem with the O star effective temperature scale and that these values may be 2000K too low. Because of this we avoided using logT_e in making the comparison with recent convective overshooting models. Y Cygni is a very young system, with an age of ∼2-3x10⁶yr. These models also provide an apsidal motion constant logk₂=-1.933 in excellent agreement with the observed value of -1.94±0.03.