  We present results of the analysis of a sample of 22 stars of spectral types
from O7 to B5 and luminosity classes I-V for which Spitzer/IRS spectra are
available. The IRS spectra of these stars are examined for signs of excess
infrared (IR) emission by comparison with stellar atmospheric spectra. We find
that the spectra of half of the studied stars are dominated by excess emission
in the far-IR, including all six super- and bright giants. In order to examine
the origin of the far-IR excess, we supplement the Spitzer data with optical
high-resolution echelle spectroscopy ($\lambda/\Delta \lambda \sim 10^5$),
near-IR high-contrast coronagraphic imaging taken with the SPHERE instrument at
VLT with a spatial resolution of 0.05", and WISE and Herschel photometry. In
the optical region, we detect various absorption and emission lines (H$\alpha$,
CIII, and NIII) irrespective of the far-IR excess. Pfund($\alpha$) and
Humphrey($\alpha$) lines are observed at the same time as the far-IR excess.
These lines are stronger in stars with far-IR excess than in stars without
excess. A scattered-light disk in the central r < 2.5" region of the far-IR
excess stars HD149404, HD151804, and HD154368 can be excluded from H band
imaging down to a 1$\sigma$ contrast of $F(r)/F_{*} \sim 10^{-6}$. The far-IR
excess is fit either by a free-free component from ionized gas as for the winds
of hot stars or a large (1pc) circumstellar dust shell. The putative dust
envelopes required to explain the excess have a visual extinction as low as a
few hundred $\mu$-mag.
