A tense affix ending in a is lengthened in the ṛgveda when the verb consists of two syllables.,
Thus विद्मा हि त्वा सत्पतिं शूर गोनाम् &c. See Rig III. 42. 6, विद्मा हि तस्य पितरम् &c. But not here देवा भवत वाजिनः, as the verb consists of more than two syllables: not also here आ देवान् वक्षि यक्षि च as the verbs do not end in अ ॥,