2001AJ....121..254R

Thirty-eight long-period (P>10 days) apparently contact binary stars discovered by the OGLE-II project in the SMC show EB-type light curves and an ``inverted'' period-color relation with longer orbital periods for redder systems. The strong light variations between eclipses can be explained within a semidetached model in which ellipsoidal variations of a large, evolved, Roche lobe-filling component dominates over eclipse effects in the systemic light changes. The model requires further spectroscopic and color-curve support before it can be fully accepted. It is noted that the dominant role of the Roche lobe-filling component in the total systemic luminosity can explain the new period-luminosity-color (PLC) relation, which has been established for the long-period EB (LP-EB) systems. We call it the PLC-β relation, to distinguish it from the Cepheid relation. Two versions of the PLC-β relation–based on the (B-V)₀ or (V-I)₀ color indices–have been calibrated for 33 systems with (V-I)₀>0.25 spanning the orbital period range of 11 to 181 days (it was found that blue systems with (V-I)₀≤0.25 do not follow the same calibration). The relations can provide maximum-light, absolute-magnitude estimates accurate to εM_V≃0.35 mag within the approximate range -3<M_V<+1. In terms of their number in the SMC, the LP-EB binaries are about 50 times less common than the Cepheids. Nevertheless, their large luminosities coupled with continuous light variations make these binaries very easy to spot in nearby galaxies, so that the PLC-β relation can offer an auxiliary and entirely independent method of distance determination to nearby stellar systems rich in massive stars.