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 absol ute-magnitude calibration of the form M-V = M-V(log P, B - V). The cal ibration covers the ranges 0.26 < (B - V)(0) < 1.14, 0.24 < P < 1.15 d ay, and 1.4 < M-V < 6.1, it is based on a solution weighted by relativ e errors in the parallaxes (2.7% to 24%). Previous calibrations have n ot been based on such a wide period and color space. and while they ha ve been able to predict Mc with sufficient accuracy for systems closel y following the well-known period-color relation, the new calibration should be able to also give good predictions for more exotic ''outlyin g'' contact binary systems. The main limitations of this calibration a re the inadequate quality of the ground-based photometric data, and th e restriction to the (B - V) index, which is more sensitive to metalli city effects than the (V - I) index; metallicities are, however, basic ally unknown for the local W UMa-type systems.