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We obtained 13 epochs of mid-infrared interferometry with the MIDI instrumentat the VLTI between April 2004 and July 2007, covering pulsation phases0.45-0.85 within four cycles. The data are modeled with a radiative transfermodel of the dust shell where the central stellar intensity profile isdescribed by a series of dust-free dynamic model atmospheres based onself-excited pulsation models. We examined two dust species, silicate and Al2O3grains. We performed model simulations using variations in model phase and dustshell parameters to investigate the expected variability of our photometric andinterferometric data. The observed visibility spectra do not show anyindication of variations as a function of pulsation phase and cycle. Theobserved photometry spectra may indicate intracycle and cycle-to-cyclevariations at the level of 1-2 standard deviations. The best-fitting model forour average pulsation phase of 0.64+/-0.15 includes the dynamic modelatmosphere M21n (T_model=2550 K) with a photospheric angular diameter of7.6+/-0.6 mas, and a silicate dust shell with an optical depth of 2.8+/-0.8, aninner radius of 4.1+/-0.7 R_Phot, and a power-law index of the densitydistribution of 2.6+/-0.3. The addition of an Al2O3 dust shell did not improvethe model fit. The photospheric angular diameter corresponds to a radius of520^+230_-140 R_sun and an effective temperature of ~ 2420+/-200 K. Ourmodeling simulations confirm that significant visibility variations are notexpected for RR Aql at mid-infrared wavelengths within our uncertainties.  We conclude that our RR Aql data can be described by a pulsating atmospheresurrounded by a silicate dust shell. The effects of the pulsation on themid-infrared flux and visibility values are expected to be less than about 25%and 20%, respectively, and are too low to be detected within our measurementuncertainties.

Mid-infrared interferometric monitoring of evolved stars - The dust shell around the Mira variable RR Aql at 13 epochs