Partner bids 3 of a suit as a slam try in that suit. How should opener respond?
Following discussions with David Owen, the way I play this is that 3NT shows a lack of enthusiasm for the slam try, any other bid is a RKCB response. So:
As 3NT is removed from the RKCB sequence, there are always four possible bids, culminating in 4 of the slam-try suit. It's clearly unlikely that opener would have cooperated with the slam try with zero keycards, one is just about possible. If responder now bids 4 of the slam-try suit, it's asking opener to pass with one or zero keycards, otherwise cue-bid. If opener shows either 1/4 or 0/3 keycards, responder can bid the next suit up to enquire about the trump queen. Holding the trump queen, opener now cue-bids his cheapest king.