  In the group of B stars with spectroscopic peculiarities, we can find the Be
and the B[e] stars. The Be stars are early-type rapid rotators that present, as
their principal characteristic, emission lines of hydrogen and singly ionized
metals due to the presence of a gaseous envelope. The B[e] stars present in
their spectra heterogeneous features that reveal the presence of regions with
very different properties in a gaseous and dusty envelope. Our goal is to study
the evolution of the disks around peculiar B stars through the variability of
their physical properties and dynamical structure, as well as to set
constraints on different models and disk forming mechanisms. Throughout the
last decade, we have carried out temporal monitoring of a sample of objects in
the near infrared using spectroscopic facilities at the Gemini and Las Campanas
Observatories. In the present work, we focus on the classical Be star 12 Vul,
for which also optical spectra have been collected quasi-simultaneously. We
observed variability in the hydrogen line profiles of 12 Vul, attributed to
dissipating and building-up processes of the circumstellar envelope. Also, we
found that this Be star presented the $^{12}$CO band heads in emission in one
observation. The emission of this molecule has not been previously reported in
a Be star, while it is a common feature among B[e] stars. We obtained
parameters to describe the $^{12}$CO emitting region and propose different
scenarios to explain this intriguing emission.
