The stage sees the dying star, initially very cool and large, shrink and heat up.
[2] Towards the end of this stage, post-AGB stars also tend to produce protoplanetary nebulae as they shed their outer layers, and this creates a large infrared excess and obscures the stars in visible light.
On the other hand, the luminosity of post-AGB stars is usually constant throughout the post-AGB stage, and slightly dependent on the star’s core mass, and getting slightly brighter with lower metallicity.
[3][2] Due to the dust usually obscuring them, many post-AGB stars are visually relatively dim.
However there are still some post-AGB stars visible to the naked eye, the brightest of which is 89 Herculis.