  Experiments involving ultracold molecules require sufficiently long
lifetimes, which can be very short for excited rovibrational states in the
molecular potentials. For alkali atoms such as rubidium, a lowest rovibrational
molecular state can both be found in the electronic singlet and triplet
configurations. The molecular singlet ground state is absolutely stable.
However, the lowest triplet state can decay to a deeper bound singlet molecule
due to a radiative decay mechanism that involves the interatomic spin-orbit
interaction. We investigate this mechanism, and find the lifetime of rubidium
molecules in the lowest triplet rovibrational state to be about 21 minutes.
