In an attempt to determine the nature of the enigmatic cocoon stars in the Quintuplet Cluster , we have obtained mid-infrared imaging and spectrophotometry of the cluster , using the CAM and SWS instruments on ISO , using SpectroCam-10 on the Palomar 5m telescope , and NICMOS on HST .
The spectra show smooth continua with various dust and ice absorption features .
These features are all consistent with an interstellar origin , and there is no clear evidence for any circumstellar contribution to these features .
We find no spectral line or feature that could elucidate the nature of these sources .
Detailed modeling of the silicate absorption features shows that they are best reproduced by the \mu Cep profile , which is typical of the interstellar medium , with \tau _ { sil } \simeq 2.9 .
The high spatial resolution mid-IR images show that three of the five cocoon stars have spatially extended and asymmetric envelopes , with diameters of \sim 20 , 000 AUs .

A reddening law similar to that of Lutz ( 1999 ) but with silicate features based on the \mu Cep profile and normalized to our value of \tau _ { sil } is used to deredden the observed spectrophotometry .
The dereddened energy distributions are characterised by temperatures of 750–925 K , somewhat cooler than determined from near IR data alone .
Models of optically thin and geometrically thick dust shells , as used by Williams et al .
( 1987 ) for very dusty , late-type WC stars , reproduce the observed SEDs from 4 to 17 \mu m , and imply shell luminosities of \log ( L / L _ { \odot } ) \simeq 4.5 –4.9 for the brightest four components .
An analysis of the various suggestions proposed to explain the nature of the cocoon stars reveals serious problems with all the hypotheses , and the nature of these sources remains an enigma .