New findings based on long-term photometric observations of the eclipsing binary V471 Tauri

Abstract

The post‐common envelope and pre‐cataclysmic binary V471 Tau has been observed by the authors since 1973. At least a complete light curve in B and V bands and more than two eclipse timings were obtained in each year. All the available data published so far (including the authors') have been collected and analysed for the brightness and orbital period changes. The system brightened about 0.22 mag in both B and V bands more or less regularly up to 1997 and started to decrease afterwards. A search for periodicity of this variation yields a period longer than 85 yr. In addition to this long‐period variation, a small amplitude of about 0.08 mag and short time‐interval fluctuations on the mean brightness have been detected. The variations of the mean brightness have been discussed and plausible causes suggested. The changes of the apparent period have been attributed to a third body. Analysis of all the ‘observed−calculated’ (O−C) data yields a period of 32.4 yr, with a semi‐amplitude of 151 s and an eccentricity of 0.30 for the third‐body orbit. For orbital inclinations greater than 34° the mass of the third body would possibly match to a brown dwarf. One of the most interesting features in the light curve of V471 Tau is the decrement of the eclipse depth with time. The depth of the eclipse in the B band has been decreased from 0.082 to 0.057 mag over 34 yr. Subtracting the variation of the depth due to the brightening of the red dwarf star, the actual variation in depth, originated from from the white dwarf, was found to be about 0.012 mag. This change in the brightness of the compact object has been attributed to the mass accretion from its primary component via thermally driven wind and/or flare‐like events.

The (O–C) diagram problem of triple pre-cataclysmic system V471 Tau solved

“…3) it is seen that our data confirm one of the assumed trends of the (O-C) diagram, namely a repeated decrease of the (O-C) data. If we compare our (O-C) diagram constructed on the basis of older ephemeris by Guinan and Ribas (2001) we can see that the last change of the trend of the (O-C) diagram occurred in higher (O-C) values than predicted by Guinan and Ribas (2001), Ibanoglu et al (2005) and Kamiński et al (2007). For better understanding, see Fig.…”

“…Moreover, due to testing of the minima we detected them also by overlay eclipse points in the wings with a straight line and we got the same results within the errors. For the construction of the (O-C) diagram we used in our paper new ephemeris 2 from the minima obtained by us, the minima obtained at our observatories in the past (Hric et al 2003) and the minima from accessible works Ibanoglu et al (2005). In the works we met with an inaccuracy of 0.003 days, which is approximately 260 seconds.…”

MOSTPhotometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau

“…5). The MOST eclipse time determination is shown compared to all available published data (as discussed by Ibanoglu et al 2005) in Figure 6. The O À C curve continues an upward trend seen for about the last 10,000 orbital cycles.…”

Origin of apparent period variations in eclipsing post-common-envelope binaries

“…Period variations have been observed for a long time (e.g., Lohsen 1974), and many possibilities were suggested in the past to explain the shape of the O−C diagram (e.g., Ibanoglu et al 1994;İbanoǧlu et al 2005;Guinan & Ribas 2001;Kamiński et al 2007), such as perturbations by a third body in a long-period orbit, apsidal motion due to a low orbital eccentricity, or even mass transfer. Recently, Kundra & Hric (2011) have detected a change in the O−C diagram trend.…”