On the X2Σ+, A2Π, and C2Σ+ states of BeH, BeD, and BeT

New Fourier transform measurements for the A2Π − X2Σ+ system of BeH are combined with previously published A − X data for BeH, BeD, and BeT, with existing data for the C2Σ+ − X2Σ+ system, and with recent vibration-rotation data for BeH and BeD, and fitted using combined-isotopologue Dunham expansion and direct-potential-fit methods. This data set provides direct spectroscopic information spanning 95% of the ground X2Σ+ state potential well, and provides the most comprehensive spectroscopic description of this state reported to date. The analysis of this data set allows us to study the breakdown of the Born-Oppenheimer approximation for a metal hydride from the minimum of the potential well to near the dissociation limit. Improved molecular constants are also determined for the A2Π and C2Σ+ states.