  We present observations of the occultation of Umbriel by Oberon on 4 May,
2007. We believe this is the first observed mutual event between satellites of
Uranus. Fitting a simple geometric model to the lightcurve, we measure the
mid-event time with a precision of 4 seconds. We assume previously measured
values for the albedos of the two satellites (Karkoschka 2001), and measure the
impact parameter to be 500 +/- 80 km. These measurements are more precise than
estimates based on current ephemerides for these satellites. Therefore
observations of additional mutual events during the 2007-2008 Uranian equinox
will provide improved estimates of their orbital and physical parameters.

Astrophysics