1994PASP..106..462R

We use the basic expression for luminosities of contact systems, L proportional to area * T_e^4, to derive calibrations of their absolute magnitudes M_V in terms of the orbital period and color indices B-V and V-I_C. These calibrations can serve as a consistency check on membership of W UMa systems in stellar clusters and will permit the use of W UMa binaries as distance indicators for stellar systems consisting of solar-type stars.

The calibrating sample at this time is based on 3 systems with trigonometric parallaxes, 11 systems in high galactic latitude open clusters and 3 systems with main-sequence visual companions. The ranges in period and color are quite narrow and the results must be considered as preliminary, especially for the V-I_C calibration.

One of the possible applications of the calibrations, as a sieve to remove the foreground interlopers, is illustrated. The result is that the cluster and field frequencies of optically detected W UMa systems seem to be equal at about one per about 280 surveyed stars (uncorrected for the inclination bias of about two times), which is about 3-4 times higher in the field than estimated before.