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 (log P ), 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 log P -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.
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