  (abridged) In this work we present the results of an optical and IR study for
the curious B[e] star CD-42 11721. This object has a doubtful evolutionary
stage, being either a pre-main sequence star (HAeB[e]) or an evolved star
(sgB[e]). Our optical investigation could be splitted in two parts, a
qualitative study based on the identification of the numerous emission lines
present in the spectra and the classification of their line profiles, which
indicate a non-spherically symmetric circumstellar environment, and a
quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [NII] and
[SII]. Assuming a typical circumstellar scenario for a sgB[e], i.e. a fast,
low-density polar wind and a slow, high-density disc forming equatorial wind,
we can reproduce very well the line luminosities of the forbidden lines. From
this analysis, we can determine the mass loss rate of the star lying in the
range from 4.4 x 10^(-6) to 2.2 x 10^(-5) M_sun/yr, depending on the considered
redenning. Our IR study could also be splitted in two parts: the identification
of several features in the SWS ISO spectrum, and the modeling of the SED of
CD-42 11721. The first part shows the presence of a mixed chemistry, i.e. C-
and O-rich dust in the same circumstellar medium. We have tried to model the
SED, by using a numerical code written by us which considers a spherical
circumstellar scenario. Although the answer concerning the evolutionary stage
of CD-42 11721 is still not very clear, we believe that our analysis will
improve the discussion about the nature of this curious star.
