Long-term eclipse timing of white dwarf binaries: an observational hint of a magnetic mechanism at work

Abstract

We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have observed more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yrs, with secondaries of spectral type K2 -M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yrs, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries.

The quest for stable circumbinary companions to post-common envelope sdB eclipsing binaries

“…• We find agreement with Bours et al (2016) that a decade or so of observations are required to establish a reliable ephemeris but we do not find a tendency for the RMS weighted residuals to saturate at ∼100. For the systems we considered we found RMS values ranging between 1.6 and 21.0.…”

“…They found no direct evidence of such an object. More recently, Bours et al (2016) carried out a long-term programme of eclipse time measurements on 67 white dwarfs in close binaries to detect period variations. They found that all systems with baselines exceeding 10 years, and with companions of spectral type M5 or earlier, appeared to show much larger eclipse timing variations than systems with companions of spectral types later than M5.…”

“…Following the methodology of Bours et al (2016), we investigated the correlation between the RMS residuals of the seven sdB binary systems reported here with their observational baselines and their spectral types. Unfortunately the spectral types of the binary companions are somewhat uncertain as these stars are not spectroscopically visible and assessment has to be made indirectly through parameters that are generally poorly constrained.…”

Hunting for eclipses: high-speed observations of cataclysmic variables

“…We supply the mid-eclipse times in Table 1 in case this information is useful to future orbital period variability studies (e.g. Parsons et al 2010;Bours et al 2016), but remind the reader that eclipse times measured by pt5m suffer from the systematic uncertainty introduced by treating the eclipses as Gaussians.…”

The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data

“…Of the planets detected through eclipse timing, the most convincing case is given by two circumbinary planets orbiting the pre-cataclysmic binary NN Ser. Eight years after the discovery paper (Qian et al 2009; see also Beuermann et al 2010) and 26 years after the first data, their existence remains the best explanation for the observed eclipse time variations (Bours et al 2016). Many other detached close binaries show eclipse time variations: for some of them, the presence of planets is excluded by dynamic stability computations and the periodic O-C trends may be caused by other effects, such as Applegate-like mechanisms (Applegate 1992;Lanza 2006).…”