[Cthulhu D]

You sit North, against two decent club players. I am unable to use the BBO hand tool. (Edit: Whoops, maybe should have put this in novice/beginner)

Dlr: E
Vul: N-S

S: T
H: 8
D: AKJT65432
C: 62

The auction proceeds:

(1S) - X* - (4S)

15-19 semi balanced or so and does not promise a stop. It's pretty much a 1NT overcall that doesn't promise a stop. Some stronger hand types are possible, but not that likely compared to the strong NT

How many diamonds do you bid?

If you bid 6D

Spoiler

This gets doubled by RHO and passed in

If you bid 5D there is a bonus question

Spoiler

The auction continues
(1S) - X* - (4S) - 5D
(5S) - X - (P) - ???

What do you do now?

Results:

Spoiler

6D is off two cashing aces, 5SX goes for 800.

[benlessard]

6D is the % bid imo. Out fo the 32 pts remaining partner got at least half of them with a bias toward non spades pts. I think hes 60%+ favorite to hold at least 2 aces. Losing 2 clubs is possible while them going with the wrong lead or take a phantom over 6D is very unlikely. But all in all I like 6D.

[rmnka447]

Bid 5 ♦. There's just no way to know if partner or opener has 2 of the 3 remaining As. The opponent's 1 ♠ - 4 ♠ preemption has done its job, so opt for the most likely positive result.

If the opponent's bid further, we'll be in a forcing pass situation as we've voluntarily bid game.

[ArtK78]

rmnka447, on 2012-May-18, 11:56, said:

Bid 5 ♦. There's just no way to know if partner or opener has 2 of the 3 remaining As. The opponent's 1 ♠ - 4 ♠ preemption has done its job, so opt for the most likely positive result.

If the opponent's bid further, we'll be in a forcing pass situation as we've voluntarily bid game.

Yes, you are in a forcing pass situation, but no, you have not "voluntarily" bid game. You bid game in a contested auction. Partner has no way of knowing what you would have done if your RHO had not bid 4♠.

[akhare]

No idea of what's right here, but I would go with 6♦. If I bid 5♦, I will be put to a second guess over the likely 5♠...

[hrothgar]

I know that folks might be shocked, but I am strongly tempted to pass

We're red, they're white
Bidding high might push them to a making slam

[Statto]

The meaning for the X is highly unusual. Even so, I don't think it's that likely partner will be able to provide 3 very quick tricks outside ♦, so I'll bid 5♦. When partner doubles 5♠, I'll take the penalty.

[Cthulhu D]

Statto, on 2012-May-23, 18:53, said:

The meaning for the X is highly unusual. Even so, I don't think it's that likely partner will be able to provide 3 very quick tricks outside ♦, so I'll bid 5♦. When partner doubles 5♠, I'll take the penalty.

I probably should have clarified in the OP that it's similar to a natural 1NT overcall that doesn't promise a stopper - it's just that we have some of the weirder hands in with our 1NT overcall rather than our takeout double (which is in 1NT).

Still, interesting consensus. My team mate had this problem and pulled partner's 5Sx (partner having AQJ tight of spades over the K), then misplayed it after they came up with a low spade lead for which he was mocked and subjected to the traditional 20 imps out on one board penalty of having to buy everyone a drink. At my table they just bid 6D directly which we couldn't double but went off when P cashed his aces. Afterwards the team was divided 50/50 on 6D or 5D, so it's interesting to see the results repeated.