What do I wear under this halterneck?

[boogeek]

In a fit of ebay enthusiasm (given that all maternity wear in the shops this season is black or white and sensible) I bought this sundress - I didn't even really notice it was halterneck. I don't really do halterneck. It is really nice, but I'd like to wear it not only on the beach and it is really low at the front and really thin cotton. I could put T-shirt under it, but it is hot; a vest top looks weird; what do I do? Please?

I would wear a bikini top underneath it, but you could also get away with a vest top if it was the thin strappy type maybe with a v neck.
HTH

a boob tube?

Thanks - bikini top could work. Can't bear the thought of a boob tube!