Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Long-term eclipse timing of white dwarf binaries: an observational hint
  of a magnetic mechanism at work   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.

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The categories of this paper are: hep-ph
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The categories of this paper are: q-fin
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The categories of this paper are: math
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The categories of this paper are: gr-qc
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The categories of this paper are: solv-int
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The categories of this paper are: hep-lat
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-th
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The categories of this paper are: hep-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: q-alg
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The categories of this paper are: chao-dyn
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The categories of this paper are: stat