  HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and
dust. Its spectrum shows characteristics which may indicate either a pre- or a
post-main-sequence system. Mapping the kinematics of the gas in the inner few
au of the system contributes to a better understanding of its physical nature.
We present the first high spatial and spectral resolution interferometric
observations of the Br-gamma line of HD~50138, obtained with VLTI/AMBER. The
line emission originates from a region more compact (up to 3 au) than the
continuum-emitting region. Blue- and red-shifted emission originates from the
two different hemispheres of an elongated structure perpendicular to the
polarization angle. The velocity of the emitting medium decreases radially. An
overall offset along the NW direction between the line- and continuum-emitting
regions is observed. We compare the data with a geometric model of a thin
Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the
data are well reproduced by this model, except for the variability, the global
offset and the visibility at the systemic velocity. The evolutionary state of
the system is discussed; most diagnostics are ambiguous and may point either to
a post-main-sequence or a pre-main-sequence nature.
