  Using a radiative transfer code (DUSTY) parameters of the circumstellar dust
shells of 15 hot post-AGB stars have been derived. Combining the optical, near
and far-infrared (ISO, IRAS) data of the stars, we have reconstructed their
spectral energy distributions (SEDs) and estimated the dust temperatures, mass
loss rates, angular radii of the inner boundary of the dust envelopes and the
distances to these stars. The mass loss rates
(10$^{-6}-10^{-5}$M$_{\odot}$yr$^{-1}$) are intermediate between stars at the
tip of the AGB and the PN phase. We have also studied the ISO spectra of 7 of
these stars. Amorphous and crystalline silicate features were observed in
IRAS14331-6435 (Hen3-1013), IRAS18062+2410 (SAO85766) and IRAS22023+5249 (LSIII
+5224) indicating oxygen-rich circumstellar dust shells. The presence of
unidentified infrared (UIR) band at 7.7$\mu$, SiC emission at 11.5$\mu$ and the
"26$\mu$" and "main 30$\mu$" features in the ISO spectrum of IRAS17311-4924
(Hen3-1428) suggest that the central star may be carbon-rich. The ISO spectrum
of IRAS17423-1755 (Hen3-1475) shows a broad absorption feature at 3.1$\mu$ due
to C$_{2}$H$_{2}$ and/or HCN which is usually detected in the circumstellar
shells of carbon-rich stars.

Astrophysics