Left panels: AAVSO and ASAS V band light curves of the six stars with pulsation-related flux variations larger than 0.5 mag. The blue dotted line is a fit to the full light curve as described in Sect. 3.3, only shown to guide the eye. The coloured vertical lines indicate the time of observation of the mid-IR spectra shown in the right panels. Thick black, thick blue and thick red lines represent data from Spitzer, ISO and Timmi2, respectively. The thick red dashed line indicates the three-filter VISIR photometry of U Mon (published by Lagadec et al. 2011). The thin coloured lines represent our MIDI measurements at various epochs. Right panels: measured mid-IR fluxes for the indicated epochs in the left panels. The minimum error over the shown wavelength range is indicated for each spectrum with a bar. For RV Tau and U Mon, a few very noisy MIDI spectra are not included in the right panel. The red dotted line in the right panel of U Mon is the orange spectrum scaled to the VISIR fluxes.