  We present new WSRT observations of the micro-arcsecond quasar J1819+3845.
All short term variations are attributed to interstellar scintillation of a
source which is at most 30 micro-arcseconds in diameter. The timescale of the
modulations changes over the year, which we interpret as due to a peculiar
velocity of the scattering medium. The scintillation behaviour can be used to
determine sub-structure in the source.
