O'Connell effect in early‐type contact binaries: DU Boo and AG Vir

Abstract

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 ΔT ≈ 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.

Starspots in contact and semi-detached binary systems

“…Additionally, most of A-type sample binaries have temperatures higher than 6000 K. Therefore, other mechanism, which differs from stellar dynamo, may forms the cool spots in A-type binaries. A different mechanism for the spot formation in A-type systems is also supported by Pribulla et al (2011). SD2 sample binaries also generally have cool spots on the components with temperatures lower than 6000 K. Nevertheless, their associations considerably differ from those of the W-type sample.…”

Bright Southern Variable Stars in the bRing Survey

“…The differences between the maxima were considered significant if they exceeded 3σ A , where σ A is the uncertainty in the amplitude of the EB. Only two of the EBs in this table (TY Men (Nagy 1985;Pribulla et al 2011) and TU Mus (Terrell et al 2003) have been noted in the literature as having evidence of asymmetry in their light curves. The other 16 have likely been missed due to the faint effect observed in bRing and possible variability of the effect The symbols for previously identified variables are outlined in black and newly identified variables are solid black.…”

“…In W UMa and β Lyr EBs, the O'Connell effect is observed as an asymmetry in the maximum brightness in between the primary and secondary eclipses, i.e., the maximum before the primary eclipse is fainter than the maximum before the secondary eclipse (O'Connell 1951). The underlying physical mechanism is not well understood (plausible explanations include surface features and Doppler beaming: Wilsey & Beaky 2009;da Silva et al 2014) though several examples have been detected (Pribulla et al 2003(Pribulla et al , 2011Burggraaff et al 2018).…”

Observations of Two Unusual Eclipsing Binaries, FN Cam and AG Vir

“…The cyclic variations are caused by the third body, a trace of which was later found by Pribulla et al(2006). Pribulla et al (2011) carried out photometric and spectroscopic observation, but failed to derive a simultaneous solution of light curves and broadening functions (BFs). They noticed that BFs of AG Vir indicates a stream of matter or a bright region.…”

“…The possible configuration of the 'hot area' is also still unknown. Pribulla et al (2011) stressed that highresolution spectroscopy near the H α or Ca II H and K lines was needed for future studies. Because of these reasons, we added this binary to our list.…”