Eclipsing Binaries in the OGLE Variable Star Catalog. IV. The Precontact, Equal-Mass Systems

We used the database of eclipsing binaries detected by the OGLE microlensing project in the pencil-beam search volume toward Baade's Window to define a sample of 74 detached, equal-mass, main-sequence binary stars with short orbital periods in the range 0.19<P<8 days. The logarithmic slope of the period distribution logN~(-0.8+/-0.2) was used to infer the angular-momentum-loss (AML) efficiency for the late, rapidly rotating members of close binaries. It is very likely that the main cause of the negative slope is a discovery selection bias that progressively increases with the orbital period length. Assuming a power-law dependence for the correction for the bias ~-ClogP (with C>=0) the AML braking-efficiency exponent α in dH/dt=P^-α can take any value α=-1.1 (+/-0.2)+C. Very simple considerations of discovery biases suggest C~=4/3, which would give an AML braking law very close to the ``saturated'' one, with no dependence on the period. However, except for plausibility arguments, we have no firm data to support this estimate of C, so that α remains poorly constrained. The results signal the utmost importance of the detection bias evaluation for variable star databases used in analyses similar to the one presented in this study.