Eclipsing binaries in the OGLE variable star catalogs. V. Long-period EB-type light curve systems in the Small Magellanic Cloud and the PLC-beta relation

Thirty-eight long-period (P < 10 days) apparently contact binary stars disc overed 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 syst ems. The strong light variations between eclipses can be explained within a semidetached model in which ellipsoidal variations of a large, evolved, Ro che lobe-filling component dominates over eclipse effects in the systemic l ight changes. The model requires further spectroscopic and color-curve supp ort 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 expla in the new period-luminosity-color (PLC) relation, which has been establish ed for the long-period EB (LP-EB) systems. We call it the PLC-<beta> relati on, to distinguish it from the Cepheid relation. Two versions of the PLC-be ta relation-based on the or color indices-have been calibrated for 33 syste ms with (V - I)(0) less than or equal to 0.25 spanning the orbital period r ange of 11 to 181 days (it was found that blue systems with (V - I)(0) less than or equal to 0.25 do not follow the same calibration). The relations c an provide maximum-light, absolute-magnitude estimates accurate to epsilonM (V) similar or equal to 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 time s less common than the Cepheids. Nevertheless, their large luminosities cou pled with continuous light variations make these binaries very easy to spot in nearby galaxies, so that the PLC-beta relation can offer an auxiliary a nd entirely independent method of distance determination to nearby stellar systems rich in massive stars.