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In an attempt to determine the nature of the enigmatic cocoon stars in theQuintuplet Cluster, we have obtained mid-infrared imaging and spectrophotometryof the cluster, using the CAM and SWS instruments on ISO, using SpectroCam-10on the Palomar 5m telescope, and NICMOS on HST. The spectra show smoothcontinua with various dust and ice absorption features. These features are allconsistent with an interstellar origin, and there is no clear evidence for anycircumstellar contribution to these features. We find no spectral line orfeature that could elucidate the nature of these sources. Detailed modeling ofthe silicate absorption features shows that they are best reproduced by the muCep profile, which is typical of the interstellar medium, with tau(sil) \sim2.9. The high spatial resolution mid-IR images show that three of the fivecocoon stars have spatially extended and asymmetric envelopes, with diametersof \sim 20,000 AUs.  A reddening law similar to that of Lutz (1999) but with silicate featuresbased on the mu Cep profile and normalized to our value of tau(sil) is used toderedden the observed spectrophotometry. The dereddened energy distributionsare characterised by temperatures of 750-925 K, somewhat cooler than determinedfrom near IR data alone. Models of optically thin and geometrically thick dustshells, as used by Williams et al. (1987) for very dusty, late-type WC stars,reproduce the observed SEDs from 4 to 17 mic, and imply shell luminosities oflog(L/L(sun)) \sim 4.5-4.9 for the brightest four components. An analysis ofthe various suggestions proposed to explain the nature of the cocoon starsreveals serious problems with all the hypotheses, and the nature of thesesources remains an enigma.Comment: 16 pages, 11 figures, A&A style. Accepted by A&

Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster