  We have studied the core morphology of the ultraluminous infrared galaxy Mrk
273 by combining a high-resolution adaptive optics near-infrared image with an
optical image from the Hubble Space Telescope and interferometric radio
continuum data, all at spatial resolutions of 150 mas or better. The
near-infrared image reveals that the nucleus has two main components, both of
which have radio counterparts. The strongest component (N) shows very similar
extended structure in the radio and near-infrared. It has a flat radio spectrum
and is resolved into a double-lobed structure (Ne; Nw), with a separation of
90\pm5 mas (70 parsec). A similar structure is detected in the near-infrared.
We identify this component as the location of the active nucleus. The second
component (SW), strong in the near-infrared but relatively weak in the radio,
is located $\sim1$ arcsecond to the southwest. We interpret this as an obscured
starburst region associated with the merger. The radio continuum images show a
third, strong, component (SE) which has previously been interpreted as a second
nucleus. However, it shows no associated optical or near-infrared emission,
suggesting that it is in fact a background source.
