NASA Astrophysics Data System (ADS)
Rucinski, Slavek M.; Maceroni, Carla
2001-01-01
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 ɛMV~=0.35 mag within the approximate range -3