Two extremely compact Hα-emitting sources have been discovered in our deep images of the Abell 539 and Abell 634 clusters of galaxies. Follow-up spectroscopy of these two unresolved sources have revealed that they are active starburst members of their respective clusters. The Hα luminosities and extreme Hα equivalent widths (of ≃ 430 Å and 1010 Å, respectively) of these sources, together with their very compact appearance, have raised the question of their origin and the triggering of such active star formation in the cluster environments. Possible triggering mechanisms are pointed out, together with possible links with the recently discovered early-type ultra-compact dwarf galaxies.