2017AJ....154..125M

Tycho-Gaia Astrometric Solution (TGAS) parallax data are used to determine absolute magnitudes M_V for 318 W UMa-type (EW) contact binary stars. A very steep (slope ≃ -9), single-parameter (logP), linear calibration can be used to predict M_V to about 0.1-0.3 mag over the whole range of accessible orbital period, 0.22 P 0.88 days. A similar calibration for the most common systems with 0.275 P 0.575 days predicts M_V values to about 0.06-0.16 mag. For orbital period values both shorter and longer than the central range, the period dependence is respectively steeper and shallower, i.e., the binaries are fainter in M_V than predicted by the whole-range linear law. The steepness of the relation for short-period systems implies important consequences for the detectability of the faintest binaries, defining the short-period cut-off of the period distribution. Although the scatter around the linear logP-fit is fairly large (0.2-0.4 mag), the current data do not support the inclusion of a B-V color term in the calibration.