Infrared imaging and spectroscopy of the Luminous Blue Variables Wra 751 and AG Car

We present ground-based infrared imaging and ISO spectroscopy of the lumino us blue variables Wra 751 and AG Car. The images show in both cases a detac hed shell with a roughly circular distribution of emission. The infrared im ages of AG Car coincide very well with the optical images. The optical (H a lpha) image of Wra 751 is different from the infrared image; the H alpha ne bula is suggested to be a scattering nebula containing cold dust particles. Fitting both the images and the spectra consistently with a 1-D radiative t ransfer model, we derive properties of their dust shells. Wra 751 is surrou nded by a dust shell with inner and outer radii of 0.17 and 0.34 pc respect ively and a dust mass of 0.017 M-circle dot. The dust shell of AG Car has i nner and outer radii of 0.37 and 0.81 pc respectively and a total dust mass of 0.25 M-circle dot. Dust mass-loss rates during the formation of the she lls are 2.7 x 10(-6) and 3.4 x 10(-5) M-circle dot yr(-1), respectively. Th e total dust mass and hence the derived dust mass-loss rates are uncertain by at least a factor of two. For AG Car, the derived dust mass and mass-los s rate are higher than previous estimates. This is mainly caused by the fac t that a contribution of very large grains (> 10 mu m) is needed to explain the flux levels at longer wavelengths. Dust models for both objects fail to explain the flux shortward of 15 to 20 mu m: a population of small warm grains, not in thermal equilibrium with t he central star is necessary to explain this excess. Similarities between d ust shells around Wolf-Rayet stars and Wra 751 and AG Car (mass, grain size population, morphology) suggest a similar formation history and imply an e volutionary connection. A similar connection with red supergiants is sugges ted on the basis of the dust composition and derived time-averagedmass-loss rates.