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In this letter, we report on dual-frequency European VLBI Network (EVN)observations of the faintest and least luminous radio cores in Seyfert nuclei,going to sub-mJy flux densities and radio luminosities around 10^19 W/Hz. Wedetect radio emission from the nuclear region of four galaxies (NGC 4051, NGC4388, NGC 4501, and NGC 5033), while one (NGC 5273) is undetected at the levelof ~100 microJy. The detected compact nuclei have rather different radioproperties: spectral indices range from steep (alpha>0.7) to slightly inverted(alpha=-0.1), brightness temperatures vary from T_B=10^5 K to larger than 10^7K and cores are either extended or unresolved, in one case accompanied bylobe-like features (NGC 4051). In this sense, diverse underlying physicalmechanisms can be at work in these objects: jet-base or outflow solutions arethe most natural explanations in several cases; in the case of the undetectedNGC 5273 nucleus, the presence of an advection-dominated accretion flow (ADAF)is consistent with the radio luminosity upper limit.

The faintest Seyfert radio cores revealed by VLBI