  We selected a sample of 24 XMM-Newton light curves (LCs) of four high energy
peaked blazars, PKS 0548-322, ON 231, 1ES 1426+428 and PKS 2155-304. These data
comprise continuous light curves of 7.67h to 18.97h in length. We searched for
possible quasi-periodic oscillations (QPO) and intra-day variability (IDV)
timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS
2155-304 which was reported elsewhere (Lachowicz et al. 2009). In the remaining
23 LCs we found hints of possible weak QPOs in one LC of each of ON 231 and PKS
2155-304, but neither is statistically significant. We found IDV timescales
that ranged from 15.7 ks to 46.8 ks in 8 LCs. In 13 LCs any variability
timescales were longer than the length of the data. Assuming the possible weak
QPO periods in the blazars PKS 2155-304 and ON 231 are real and are associated
with the innermost portions of their accretion disk, we can estimate that their
central black hole masses exceed 1.2 $\times$ 10$^{7}$ M$_{\odot}$. Emission
models for radio-loud active galactic nuclei (AGN) that could explain our
results are briefly discussed.
