  The MIPSGAL 24 $\mu$m Galactic Plane Survey has revealed more than 400
compact-extended objects. Less than 15% of these MIPSGAL bubbles (MBs) are
known and identified as evolved stars. We present Spitzer observations of 4 MBs
obtained with the InfraRed Spectrograph to determine the origin of the mid-IR
emission. We model the mid-IR gas lines and the dust emission to infer physical
conditions within the MBs and consequently their nature. Two MBs show a
dust-poor spectrum dominated by highly ionized gas lines of [\ion{O}{4}],
[\ion{Ne}{3}], [\ion{Ne}{5}], [\ion{S}{3}] and [\ion{S}{4}]. We identify them
as planetary nebulae with a density of a few 10$^3\ \rm{cm^{-3}}$ and a central
white dwarf of $\gtrsim 200,000$ K. The mid-IR emission of the two other MBs is
dominated by a dust continuum and lower-excitation lines. Both of them show a
central source in the near-IR (2MASS and IRAC) broadband images. The first
dust-rich MB matches a Wolf-Rayet star of $\sim 60,000$ K at 7.5 kpc with dust
components of $\sim170$ and $\sim1750$ K. Its mass is about $10^{-3}\
\rm{M_\odot}$ and its mass loss is about $10^{-6}\ \rm{M_\odot/yr}$. The second
dust-rich MB has recently been suggested as a Be/B[e]/LBV candidate. The gas
lines of [\ion{Fe}{2}] as well as hot continuum components ($\sim300$ and
$\sim1250$ K) arise from the inside of the MB while its outer shell emits a
colder dust component ($\sim75$ K). The distance to the MB remains highly
uncertain. Its mass is about $10^{-3}\ \rm{M_\odot}$ and its mass loss is about
$10^{-5}\ \rm{M_\odot/yr}$.
