  Massive stars expel large amounts of mass during their late evolutionary
phases. We aim to unveil the physical conditions within the warm molecular
environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which
are known to be embedded in circumstellar shells and disks. We present K-band
spectra of two B[e]SGs from the Large Magellanic Cloud and four Galactic YHGs.
The CO band emission detected from the B[e]SGs LHA 120-S 12 and LHA 120-S 134
suggests that these stars are surrounded by stable rotating molecular rings.
The spectra of the YHGs display a rather diverse appearance. The objects 6 Cas
and V509 Cas lack any molecular features. The star [FMR2006] 15 displays
blue-shifted CO bands in emission, which might be explained by a possible close
to pole-on oriented bipolar outflow. In contrast, HD 179821 shows blue-shifted
CO bands in absorption. While the star itself is too hot to form molecules in
its outer atmosphere, we propose that it might have experienced a recent
outburst. We speculate that we currently can only see the approaching part of
the expelled matter because the star itself might still block the receding
parts of a (possibly) expanding gas shell.
