Early-type contact systems in the LMC MACHO database

Abstract

Two sub-samples of blue, luminous contact systems from among 86 systems discovered in LMC by the MACHO project and classified as EB3 have been analyzed: a sub-sample of 29 extremely-blue systems (XB) with the observed colours V-R_C<-0.1 and a sub-sample of 36 moderately-blue (MB) systems with -0.1<V-R_C<0. To be so blue, the XB systems must be intrinsically very hot and almost unreddened so that lack of information on reddening is unimportant for them; their properties offer us a first-time insight into the absolute-magnitude calibration for massive contact binaries. It has been found that the LMC systems are apparently not limited by the blue, short-period envelope in the colour-period diagram which had been established on the basis of systems observed in the solar neighborhood, in galactic open clusters and in the sample of Old Disk systems in the direction of the Galactic Bulge. The period - magnitude correlation for the XB systems has a similar slope to those established in the absolute-magnitude, B-V and V-I_C based, calibrations for W~UMa-type systems.