  We report the results of our search for magnetic fields in a sample of 16
field Be stars, the binary emission-line B-type star upsilon Sgr, and in a
sample of fourteen members of the open young cluster NGC3766 in the Carina
spiral arm. The sample of cluster members includes Be stars, normal B-type
stars and He-strong/He-weak stars. Nine Be stars have been studied with
magnetic field time series obtained over ~1 hour to get an insight into the
temporal behaviour and the correlation of magnetic field properties with
dynamical phenomena taking place in Be star atmospheres. The
spectropolarimetric data were obtained at the European Southern Observatory
with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We
detect weak photospheric magnetic fields in four field Be stars, HD62367, mu
Cen, o Aqr, and epsilon Tuc. The strongest longitudinal magnetic field,
<B_z>=117+-38G, was detected in the Be star HD62367. Among the Be stars studied
with time series, one Be star, lambda Eri, displays cyclic variability of the
magnetic field with a period of 21.12min. The binary star upsilon Sgr, in the
initial rapid phase of mass exchange between the two components with strong
emission lines in the visible spectrum, is a magnetic variable star, probably
on a timescale of a few months. The maximum longitudinal magnetic field
<B_z>=-102+-10G at MJD54333.018 was measured using hydrogen lines. The cluster
NGC3766 seems to be extremely interesting, where we find evidence for the
presence of a magnetic field in seven early B-type stars out of the observed
fourteen cluster members.
