SIMBAD references

We present the sub-mm variability of two of the most well-studied AGB stars, IRC + 10216 and o Ceti. The data are obtained at 450 and 850µm as part of pointing calibration observations for the James Clerk Maxwell Telescope's SCUBA-2 instrument over a span of 7 yr. The periods are derived using non-parametric methods, GATSPY SUPERSMOOTHER and P4J, in order not to assume an underlying shape to the periodicity. These were compared to two Lomb-Scargle parametric methods. We find that for both sources and wavelengths the periods derived from all methods are consistent within 1σ. The 850µm phase folded light curves of IRC + 10216 show a time lag of ∼540 d compared to its optical counterpart. We explore the origins of the sub-mm variability and the phase lag using radiative transfer models. Combining the modelling with findings in the literature, we find that the sub-mm emission and phase lag can be partially attributed to the dust formation or destruction cycle. A second, unknown mechanism must be invoked; we defer an investigation of the origin and nature of this mechanism to a future work.