  In the time domain, the radio sky in particular along the Galactic plane
direction may vary significantly because of various energetic activities
associated with stars, stellar and supermassive black holes. Using multi-epoch
Very Large Array surveys of the Galactic plane at 5.0 GHz, Becker et al. (2010)
presented a catalogue of 39 variable radio sources in the flux density range
1-70 mJy. To probe their radio structures and spectra, we observed 17 sources
with the very-long-baseline interferometric (VLBI) imaging technique and
collected additional multi-frequency data from the literature. We detected all
of the sources at 5 GHz with the Westerbork Synthesis Radio Telescope, but only
G23.6644-0.0372 with the European VLBI Network (EVN). Together with its decadal
variability and multi-frequency radio spectrum, we interpret it as an
extragalactic peaked-spectrum source with a size of <~10 pc. The remaining
sources were resolved out by the long baselines of the EVN because of either
strong scatter broadening at the Galactic latitude <1 deg or intrinsically very
extended structures on centi-arcsec scales. According to their spectral and
structural properties, we find that the sample has a diverse nature. We notice
two young H II regions and spot a radio star and a candidate planetary nebula.
The rest of the sources are very likely associated with radio active galactic
nuclei (AGN). Two of them also displays arcsec-scale faint jet activity. The
sample study indicates that AGN are commonplace even among variable radio
sources in the Galactic plane.
