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tokens: indices in the respective bands (see equation 2 ), for a large control sample of aperiodic quasars. This requires the availability of spectra in both bands, as well as optical and UV time-domain data (ideally sampled in both bands at the same time). Unfortunately, the number of quasars with such data are limited; Charisi et al. ( 2018 ) analysed a small sample of 42 quasars and found that the Doppler signature can arise by chance in (40 per cent) of the cases for the near-UV (far-UV) band. This probability reflects the limited data quality of the control sample (as is also demonstrated from the increase of chance coincidence in the lower quality far-UV data), and represents only an upper limit on how frequently quasars mimic the Doppler colour variations. A larger sample of quasars with better UV + optical is needed to assess this caveat more accurately. 4.6 Constraints from future observations An interesting question to ask is how much the evidence for or against the Doppler model may tighten with continued multiband monitoring of PG1302-102. To address this question, we generated hypothetical data representing future observations over ∼10 yr, covering two additional cycles of periodicity, and computed the p -values of the Doppler model with the extended data. For simplicity, we focused on the V and M 2 bands only. In particular, we assumed that the follow-up monitoring will continue with observations similar to those in Cycles 13 and 14. We mimicked the cadence of the existing Swift observations, by picking consecutive epochs of future observed dates that are 122 d apart (i.e. three epochs per year), and generated a total of 33 new mock observations to cover a baseline of 10 yr. We also approximated the photometric errors for the mock V - and M 2-band data by taking the average of the errors over the last seven Swift observations, which yields ∼0.03 and ∼0.01 mag, respectively. 14 Finally, we combined the additional hypothetical data with the original nine Swift points to construct the full new light curves. First, we assumed that the Doppler boost model is, in fact, correct. We simulated 50 random realizations of mock V -and M 2-band data, using the fiducial DB and DRW parameters from DHS15 . We generated continuous DB and DRW light curves for a time period of 10 yr, downsampled these at the forecast future MJDs and added photometric uncertainty. We show an example of a hypothetical light curve in the left most panel of Fig. 11 . The other two panels in Fig. 11 represent different models from the fiducial, which will be explained later in this section. We then computed the corresponding p -values of the mock data, following the method described in Section 2.2 . We generated 1000 mock light curves to assemble the reference CDF, corresponding to the ‘theoretical prediction’ for the new data in the Doppler model (see Fig. 2 ). In this analysis, we added the new mock data points one-by-one, and re-computed the p -values as each of the 1, 2,