Millimeter-VLBI observations of low-luminosity active galactic nuclei
  with source-frequency phase-referencing
  We report millimeter-VLBI results of low-luminosity active galactic nuclei (M
84 and M 87) up to 88 GHz with source-frequency phase-referencing observations.
We detected the weak VLBI core and obtained the first image of M 84 at 88 GHz.
The derived brightness temperature of M 84 core was about 7.2$\times$10$^9$ K,
which could serve as a lower limit as the core down to 30 Schwarzschild radii
was still un-resolved in our 88 GHz observations. We successfully determined
the core-shifts of M 87 at 22-44 GHz and 44-88 GHz through source-frequency
phase-referencing technique. The jet apex of M 87 could be deduced at about 46
$\mu$as upstream of the 43 GHz core from core-shift measurements. The estimated
magnetic field strength of the 88 GHz core of M 87 is 4.8$\pm$2.4 G, which is
at the same magnitude of 1-30 G near the event horizon probed by the Event
Horizon Telescope.
