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 [ O iv ] , [ Ne iii ] , [ Ne v ] , [ S iii ] and [ S iv ] .
We identify them as planetary nebulae with a density of a few 10 ^ { 3 } { 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 \sim 170 and \sim 1750 K. Its mass is about 10 ^ { -3 } { M _ { \odot } } and its mass loss is about 10 ^ { -6 } { M _ { \odot } / yr } .
The second dust-rich MB has recently been suggested as a Be/B [ e ] /LBV candidate .
The gas lines of [ Fe ii ] as well as hot continuum components ( \sim 300 and \sim 1250 K ) arise from the inside of the MB while its outer shell emits a colder dust component ( \sim 75 K ) .
The distance to the MB remains highly uncertain .
Its mass is about 10 ^ { -3 } { M _ { \odot } } and its mass loss is about 10 ^ { -5 } { M _ { \odot } / yr } .