1 Introduction

Cassiopeia A (Cas A) is the youngest SuperNova Remnant (SNR) known in our galaxy. The date of the SN explosion which has given birth to the SNR is estimated to be 1680 (Fesen et al. 1988). The SN was subluminous and/or heavely obscured, since it was not noticed, except perhaps by Flamsteed (Ashworth 1980). The progenitor of the SN was a massive star (Vink et al. 1996; Jansen et al. 1988; Fabian et al. 1980), probably of Wolf-Rayet type (Fesen et al. 1988). Freshly synthesized SN material is observed in the form of Fast Moving Knots (FMK's) (Baade & Minkowsky 1954; Chevalier & Kirshner 1979; van den Bergh & Kamper 1985 and earlier papers). There are several evidences indicating that part of the SN ejecta has condensed into dust (Lagage et al. 1996; Arendt et al. 1999, hereafter ADM99, Douvion et al. 1999, hereafter DLC99). In this paper, after having presented in Sect. 2 the observational material obtained with the Infrared Space Observatory (ISO) (Kessler et al. 1996), we discuss, in Sect. 3, the dust composition and, in Sect. 4, the dust heating.