The Russians are looking at some significant radar cross section (RCS) reductions on their Su-27 series fighters.

This is not full up stealth, you need to start at the drawing board to do that, but this is more along the line of the “Have Glass” mods done on F-16s, which involved shrouding the mechanicals of the radar systems with radar absorbent materials (RAM), using a metalized canopy to keep radar from reflecting off the cockpit contents, and some minor shaping and coatings on the engine inlets.

It’s more about making active jamming more effective, particularly in the shorter wavelengths, than it is about making an aircraft “invisible”.
The measures appear to be:

• Masking the actuators of the radar.
• Possible plasma cloaking of the radar.
• This is not as outlandish as it sounds. Using plasma to cloak a complete airframe in a moving airstream is a big deal, using it under the radome, and turning it on and off to accommodate radar scans, seems more reasonable.
• Coating the first stage fan with RAM.
• A metalized cockpit canopy.
• Spray on RAM on the external stores.