The problem of branch line-strengths in the a³∏–X¹Σ⁺ intercombination transition is addressed taking into account both spin–orbit and orbit–rotation perturbations. The theoretical analysis shows that the rotational line intensities are governed by two vibronic moments due to spin–orbit interaction and four weaker rovibronic moments attributable to orbit–rotation effects. On the experimental side, the relative rotational line intensities in the 0–0 band of the a³∏–X¹Σ⁺ transition of GaH are measured and the new line-strength expressions applied in the data analysis to evaluate some of the intensity parameters.