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tokens: map maximum. The variability period is illustrated by the red dashed line in the global spectra. A possible aliasing period is displayed in blue. Lower panel: Lomb–Scargle periodogram for the same LC. The main peak labelled ‘A’ corresponds to the period of 18.86 d, whereas ‘B’ indicates the 39 d period. 3.3 KIC 4829241, KIC 10420021 and KIC 11822535 The stars KIC 4829241, KIC 10420021 and KIC 11822535 were classified as non-periodic by Maoz et al. ( 2015 ). Instead, our study suggests a periodic or quasi-periodic modulation well describing a variability signature in the wavelet representations. We attribute to this variability periodicities ranging between 11 and 16 d, which is reasonable for WDs (e.g. Kawaler 2015 ; Valeev et al. 2017 ; Braker et al. 2017 ). In fact, these periodicities are more persistent in KIC 4829241 and KIC 10420021 than in KIC 11822535, even though also noticeable in the latter star. We do not expect to find magnetic cool-spot signatures for KIC 4829241 and KIC 11822535 considering their high T eff and their log g values (Tremblay et al. 2015 ). However, a transient signature for KIC 10420021 is conceivable in view of its T eff and log g being close to the limit value (Tremblay et al. 2015 ), so the presence of a convection zone at the WD surface can still be envisaged. Combining the surface gravities and the effective temperatures, derived spectroscopically by Hallakoun et al. ( 2018 ), with evolutionary grids (Benvenuto Althaus 1999 ), we are able to estimate the mass of these three WDs. KIC 4829241 is a WD of mass . The Lomb–Scargle periodogram for this WD shows a period of 16.68 d with an amplitude of 286 ppm, as indicated by the letter A in the upper-right panel of Fig. 5 . Following Maoz et al. ( 2015 ), given its mass, rotation combined with an accretion of gas from the interstellar medium (ISM) and the material carried on the magnetic poles generating hotspots, with an accretion luminosity of Δ L acc ∼ 1.4 × 10 29 erg s −1 , could explain the periodic modulation. Fig. 5 (upper-left panel) displays the wavelet analysis, from which we noticed a long-lasting periodicity of 16.04 d over the time observation. With a lower amplitude, we also found a 40-d period that could be considered as a possible aliasing period. The 16-d period characterizes the WD photometric modulation since it is the main period obtained in both Fourier- and wavelet-based techniques. Based on a rough FAP estimation (see Section 2 ), these periods likely lie above the confidence level. Figure 5. Left: light curves with additional treatment, wavelet local and global spectra of Kepler WDs (from top to bottom) KIC 4829241, KIC 10420021, and KIC 11822535. Contour levels are 90 , 80 , 70 , ..., 20 , and 10 of the map maximum. The variability periods are illustrated by the red dashed lines in the global spectra. Possible aliasing periods are displayed in blue. Right: respective Lomb–Scargle periodogram for the same WD. The main peak labelled 'A'