The radio source M81* at the core of the nearby spiral galaxy M81 is a low-luminosity active galactic nucleus. The close distance of 3.63Mpc allows its morphology to be studied in great detail. Here we present preliminary results from continuum 7 mm VLBI observations of its core, using phase-referencing techniques. These observations set constrains on the size of M81* at this frequency and enable us to test the frequency dependence on its physical properties.
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