In the leṭ (Vedic Subjunctive) ai is the substitute of ā in the first and second person dual of the ātmanepada ,
Thus मन्त्रयैते, मन्त्रयैथे, करवैते, करवैथे । Why is not the augment आट् changed into ऐ ? Because otherwise the rule enjoining आट् would be superfluous.,