Abstract
The 0.79 eV system is examined in absorption for the frst time and is found to consist of four lines between 0.79 and 0.8 eV. Uniaxial stress measurements on these four lines are reported and the symmetry of the centre is established as monoclinic I. The relative intensities of the stress-split components vary with stress as a result of stress-induced mixing of the excited electronic states.