Absolute-magnitude calibration for the W UMa-type systems based on HIPPARCOS data

HIPPARCOS parallax data for 40 contact binary stars of the W UMa-type (with epsilon M-V < 0.5) are used to derive a new, (B - V)-based absolute-magnitude calibration of the form M-V = M-V(log P, B - V). The calibration covers the ranges 0.26 < (B - V)(0) < 1.14, 0.24 < P < 1.15 day, and 1.4 < M-V < 6.1, it is based on a solution weighted by relative errors in the parallaxes (2.7% to 24%). Previous calibrations have not been based on such a wide period and color space. and while they have been able to predict Mc with sufficient accuracy for systems closely following the well-known period-color relation, the new calibration should be able to also give good predictions for more exotic "outlying" contact binary systems. The main limitations of this calibration are the inadequate quality of the ground-based photometric data, and the restriction to the (B - V) index, which is more sensitive to metallicity effects than the (V - I) index; metallicities are, however, basically unknown for the local W UMa-type systems.