*V471 Tau C again?

The third body around V471 Tauri seems returning...

The Mystery of the Invisible Brown Dwarf Companion to the Eclipsing Binary V471 Tauri—Analysis of 45 Years of Eclipse Timings Including K2

Free-access version of the paper here.

Quote :

The Hyades eclipsing binary (EB) V471 Tauri (BD +16°516) has been under intense study since its discovery by Nelson & Young (1970). V471 Tau is a detached EB (P = 12.51 hr) that consists of a hot DAZ white dwarf and a chromospherically active, spotted K2V star (Guinan & Sion 1984). V471 Tau is an important astrophysical laboratory for deciphering binary star evolution, particularly that of postcommon envelope binary (CEB) and pre-cataclysmic variable star systems, and providing fundamental information on white dwarfs and studying the magnetic activity and dynamos of cool stars (Guinan et al. 1986). The analyses of eclipse timings show periodic long-term (30+ year) sinusoidal-like variations in the observed minus computed (O–C) values. These eclipse timing variations (ETVs) have been interpreted as a "Light Travel Time Effect" (LTTE) arising from gravitational perturbations by a tertiary companion (e.g., Guinan & Ribas 2001; Hric & Kundra 2012; Vaccaro et al. 2015). The detailed study by Vaccaro et al. (2015) returns a period of P3 = 30.1 years, e3 = 0.39 and f (m) = 1.17 × 10−5(M⊙). Adopting an EB total mass of M1+2 = 1.875 M⊙, yields a mass of the tertiary component of M3(sin i3) = 0.035 M⊙—a mass appropriate for a brown dwarf if i3 > 30 deg.

We report on the analysis of over ~45 years of eclipse-timings including nearly 80 timings determined from short-cadence (~60 s) K2 mission photometry. The K2 observations were secured on 60 continuous days during 2016 February–April. A representative 5 day sample of the photometry is shown in Figure 1 (top panel) and below it a phased light curve is plotted. The light curves show asymmetries and ~50 minutes duration occultation-eclipses of the white dwarf. The observations were normalized and de-trended (example shown in the figure). The exonailer Python program (Espinoza et al. 2016) was used to determine the mid-eclipse times with precisions of ~5–8 s. The mean-weighted mid-eclipse timing from the K2 data is BJD 2457085.20090 with a corresponding O–C = +90.5 ± 1.5 s. The K2 timings were combined with the previous timings to calculate (O–C) points and carry out LTTE (Keplerian-orbital) least-squares fits. The (O–C) data are shown in the bottom panel of Figure 1 along with the computed system properties. The LTTE analysis of the timings returns a (O–C) semi-amplitude of 150.8 ± 1.8 s, P3 = 35.3 ± 0.4 years and e3 = 0.11 ± 0.02. Adopting the EB mass from Vaccaro et al. (2015), yields a minimum-mass of M3 > 0.043 M⊙ and semimajor axis (a3) ≈ 13.3 au. As shown in the figure, after the 35.3 years LTTE variation is removed, additional small O–C oscillations are present with a semi-amplitude of ~20 ± 2 s and a P ≈ 9.7 ± 1.2 years. These small oscillations could arise from an additional 4th body or from an Applegate-type mechanism (Applegate 1992) as discussed below.

Hmmmmm... would still like a more detailed explanation for why the BD hasn't shown up, other than an assertion that it might be a bit weird because it was subject to mass outflows from the white dwarf progenitor.

Edited to add: On the other hand, the paper of Vaccaro et al. (2015) does give a couple of reasons it could have evaded detection, so who knows.

Here it comes again...

X-Ray and Ultraviolet Observations of the Eclipsing Binary V471 Tauri with XMM-Newton: X-Ray-Cycles, Eclipse Timings and Further Evidence of a Substellar Tertiary Companion

Quote :

Few stars offer as much toward understanding fundamental aspects of stellar astrophysics and binary star evolution than the eclipsing binary V471 Tauri (DAZ+K2V; P = 12.52 hr). V471 Tauri (Tau) is the product of common-envelope binary evolution and a pre-cataclysmic variable. X-ray and UV (291 nm) observations were secured with XMM-Newton over ~ 62 ks during 2019 September. X-ray and UV photometry show variations over the binary's orbit as well as flares. A precise eclipse timing was obtained from the UV photometry with the Optical Monitor. The timing was added to prior measures and fit with a 35.6 yr Light-Travel-Time-Effect (LTTE) Keplerian orbit of a substellar, tentative brown dwarf companion. However, this companion was not detected by VLT/SPHERE. The LTTE third-body solution appears reliable, now covering ~50 yr of timings. The non-detection of the companion may be due to changes in its physical properties during the high-mass loss common envelope stage.

But no imaging detection yet.

A new paper about this elusive system
Variability of eclipse timing:the case of V471 Tauri

Quote :

The post-common envelope binary V471 Tauri has been an object of interest for decades. V471 Tau shows various phenomena due to its evolutionary state and unique properties, e.g. its magnetic accretion and eclipse timing variation (ETV). Previous authors explained the ETVs by different, sometimes contradictory theories. In this paper, we present and analyse the variability of the eclipse timing of this star. We observed V471 Tauri over the last ten years and covered the second cycle of its period variation. Based on our analysis of the presented data, we assess the possible existence of a brown dwarf in this system and derive its orbital parameters. We compare the results of our dynamical modelling to the solution predicted by Applegate-mechanism theories, which have been developed in recent studies. We found that the observed ETV cannot be explained only by the presence of additional components to the binary.

The circumbinary third body model still holds up.