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We present a long-term programme for timing the eclipses of white dwarfs inclose binaries to measure apparent and/or real variations in their orbitalperiods. Our programme includes 67 close binaries, both detached andsemi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfssecondaries. In total, we have observed more than 650 white dwarf eclipses. Weuse this sample to search for orbital period variations and aim to identify theunderlying cause of these variations. We find that the probability of observingorbital period variations increases significantly with the observationalbaseline. In particular, all binaries with baselines exceeding 10 yrs, withsecondaries of spectral type K2 -- M5.5, show variations in the eclipse arrivaltimes that in most cases amount to several minutes. In addition, among thosewith baselines shorter than 10 yrs, binaries with late spectral type (>M6),brown dwarf or white dwarf secondaries appear to show no orbital periodvariations. This is in agreement with the so-called Applegate mechanism, whichproposes that magnetic cycles in the secondary stars can drive variability inthe binary orbits. We also present new eclipse times of NN Ser, which are stillcompatible 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 baselinefor many of the binaries in the eclipse timing programme, and therefore cannotyet draw robust conclusions about the cause of orbital period variations inevolved, white dwarf binaries.

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