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Aims. We present photometric data of debris disks around HIP 103389 (HD199260), HIP 107350 (HN Peg, HD206860), and HIP 114948 (HD 219482), obtained inthe context of our Herschel Open Time Key Program DUNES (DUst around NEarbyStars). Methods. We used Herschel/PACS to detect the thermal emission of thethree debris disks with a 3 sigma sensitivity of a few mJy at 100 um and 160um. In addition, we obtained Herschel/PACS photometric data at 70 um for HIP103389. Two different approaches are applied to reduce the Herschel data toinvestigate the impact of data reduction on the photometry. We fit analyticalmodels to the available spectral energy distribution (SED) data. Results. TheSEDs of the three disks potentially exhibit an unusually steep decrease atwavelengths > 70 um. We investigate the significance of the peculiar shape ofthese SEDs and the impact on models of the disks provided it is real. Ourmodeling reveals that such a steep decrease of the SEDs in the long wavelengthregime is inconsistent with a power-law exponent of the grain size distribution-3.5 expected from a standard equilibrium collisional cascade. In contrast, avery distinct range of grain sizes is implied to dominate the thermal emissionof such disks. However, we demonstrate that the understanding of the data offaint sources obtained with Herschel is still incomplete and that thesignificance of our results depends on the version of the data reductionpipeline used. Conclusions. A new mechanism to produce the dust in thepresented debris disks, deviations from the conditions required for a standardequilibrium collisional cascade (grain size exponent of -3.5), and/orsignificantly different dust properties would be necessary to explain thepotentially steep SED shape of the three debris disks presented. (abridged)

A peculiar class of debris disks from Herschel/DUNES - A steep fall off in the far infrared