  Many galactic B[e] stars suffer from improper distance determinations, which
make it difficult to distinguish between a pre- and post-main sequence
evolutionary phase on the basis of luminosity arguments. In addition, these
stars have opaque circumstellar material, obscuring the central star, so that
no detailed surface abundance studies can be performed. We propose a different
indicator for the supergiant status of a B[e] star, based on the enrichment of
its circumstellar matter by 13C, and detectable via its 13CO band emission in
the K band spectra. Based on stellar evolution models, we calculate the
variation of the 12C/13C isotopic surface abundance ratio during the evolution
of non-rotating stars with different initial masses. For different values of
the 12C/13C ratio we then compute synthetic first-overtone vibration-rotational
band spectra from both the 12CO and 13CO molecule at different spectral
resolutions. We further discuss the influence of stellar rotation on the
variation of the surface 12C/13C ratio. The surface 12C/13C isotope ratio is
found to decrease strongly during the post-main sequence evolution of
non-rotating stars, from its interstellar value of about 70 to a value of about
15-20 for stars with initial masses higher than 7 M_sun, and to a value of less
than 5 for stars with initial masses higher than 25 M_sun. We find that
detectable 13CO band head emission is produced for isotope ratios 12C/13C < 20,
and can most easily be detected with a spectral resolution of R ~ 1500...3000.
For the rotating stellar models, the drop in 12C/13C already occurs for all
stars with M > 9 M_sun during the main-sequence evolution. The detection of
13CO band head emission in such mid-resolution K band spectra of a B[e] star
thus favours an evolved rather than a young nature of the object.
