  A robust classification of Cepheids into their different sub-classes and, in
particular, between classical and Type II Cepheids, is necessary to properly
calibrate the period-luminosity relations and for populations studies in the
Galactic disc. Type II Cepheids are, however, very diverse, and classifications
based either on intrinsic (period, light curve) or external parameters (e.g.,
[Fe/H], |z|) do not provide a unique classification. We want to ascertain the
classification of two Cepheids, HQ Car and DD Vel, that are sometimes
classified as classical Cepheids and sometimes as Type II Cepheids. To achieve
this goal, we examine both their chemical composition and the presence of
specific features in their spectra. We find emission features in the H{\alpha}
and in the 5875.64 {\AA} He I lines that are typical of W Vir stars. The
[Na/Fe] (or [Na/Zn]) abundances are typical of thick-disc stars, while BL Her
stars are Na-overabundant ([Na/Fe]>+0.5 dex). Finally, the two Cepheids show a
possible (HQ Car) or probable (DD Vel) signature of mild dust-gas separation
that is usually observed only in long-period type II Cepheids and RV Tau stars.
These findings clearly indicate that HQ Car and DD Vel are both Type II
Cepheids from the W Vir sub-class. Several studies have reported an increase in
the Cepheids' abundance dispersion towards the outer (thin) disc. A detailed
inspection of the Cepheid classification, in particular for those located in
the outer disc, will indicate whether this feature is real or simply an
artefact of the inclusion of type II Cepheids belonging to the thick disc in
the current samples.
