O'Connell effect in early-type contact binaries: du Boo and AG Vir

The paper presents new photoelectric observations of the eclipsing binary systems DU Boo and AG Vir. The systems are somewhat similar - both are A-type contact binaries with the maximum following the primary minimum being the brighter one. This light curve asymmetry is extremely temporally stable. The phase dependence of the color indices is unexpectedly small for the observed amplitude of the O'Connell effect, amounting to about 0.1 mag in the optical wavelength range which indicates a very large heated area with a temperature contrast of {Δ T ≈} 1000-1500 K. The broadening functions (BFs) of the systems do not show any dark solar-type photospheric spots. On the other hand, there are significant differences of BFs between the quadratures (surprisingly similar in both systems) indicative of stream of matter or bright region causing additional emission seen between the components around the phase 0.25. Absolute parameters of the components slightly depend on the adopted model. Long orbital period of both contact binaries combined with late spectral type indicate that the primary components of either of the systems (but particularly in case of DU Boo) already evolved off the main sequence.

Based on spectroscopic data from the David Dunlap Observatory, University of Toronto