From Gaia Science Alerts Working Group
Long period variables (LPVs)
LPVs are a class of variability characteristic for evolved stars of low and intermediate mass. Most of them are on the Asymptotic Giant Branch (AGB). Being intrinsically very luminous Gaia will measure them in a large space volume.
The LPV type variability is characterized by light variations reaching an amplitude of up to 6 mag in the visual range, but also stars with light amplitudes below 1 mag belong to this group. They all show light variations on time scales between 30 and 1000 days mixed with phases of irregularity. For the Gaia Science Alert system the critical case is a subgroup, normally dubbed "miras", with amplitudes of several magnitudes and periods of a few hundred days. Due to their large amplitudes some of these stars will fall below the Gaia detection limit during their light minimum. When the star's brightness rises again, it would suddenly appear in the Gaia field as a "new" star and might trigger a science alert, especially as the star may continue to increase in brightness for several weeks.
Possible strategies to identify such events: first, these stars are very red. Including a colour criterion may help to identify these targets. Typical effective temperature is below 4000 K (corresponding BP-RP colours have to be added later). Once the star has been processed by the variability analysis (CU7) a check of the variability class should avoid any further misinterpretation (i.e. problem occurs only at the first detection). Finally, the increase in brightness is comparably slow - about 1 mag in 25 days, although there are also cases known with a very steep increase in brightness (highly asymmetric light curve).