  We have produced ultracold heteronuclear YbRb$^*$ molecules in a combined
magneto-optical trap by photoassociation. The formation of electronically
excited molecules close to the dissociation limit was observed by trap loss
spectroscopy in mixtures of $^{87}$Rb with $^{174}$Yb and $^{176}$Yb. The
molecules could be prepared in a series of vibrational levels with resolved
rotational structure, allowing for an experimental determination of the
long-range potential in the electronically excited state.
