Chromatic evolution of the DDI binaries during their phase of rapid brightening in the Gaia bands. Panel a shows the change in Gaia magnitude and colour during this phase, with the origin representing the point of minimum luminosity. The colours denote the ZAMS mass of the donors, as in the legend. Panel b shows the fraction of DDI binaries for which photometric changes are expected to be detectable by Gaia as a function of distance –neglecting effects of extinction– in each Gaia photometric band, coloured accordingly as in the legend. The vertical black lines indicate, from left to right, the approximate distance to the galactic centre, the far edge of the Milky Way (based on its D₂₅-isophotal diameter Goodwin et al. 1998), and the large and small Magellanic clouds.