what house work are au pairs supposed to help with?

Mine hangs the washing out after I have gone to work, hoovers downstairs twice a week and does the ironing during the week (I do it at weekends).

I think it is important to spell out before they come what they will need to do.

I also expect her to muck in with our sons when it comes to clearing the table after dinner.

My sister in law has an au pair to just clean.

Personally i wouldn't want to clean for 5 hours a day but she doesn't seem to have a problem recruiting or keeping au pairs.

The au pairs do all of the cleaning, washing and ironing

Depends on your individual agreement with the AP. I would say that any general housework would be ok as long as you agreed it with au pair before she turns up. (Nannys on the other hand, tend to only do child-related laundry/tidying etc).
My AP does hoovering, dusting, wiping down kitchen worktops, mopping kitchen floors, but I wouldn't ask her to clean bathrooms/toilets and she only does communal areas of the house, not bedrooms. I also don't ask for laundry or ironing or cooking. (but that's because that is what I want, not because you can't ask an au pair to do those things).

Generally they can be asked to help out with household chores that involve the kids. In my case the au pairs cooks some meals for the kids and is expected to leave the kitchen tidy (for a given value of tidy !) afterwards, and to help out with the kids laundry and ironing.

However, they should not be expected to do general housework, and I make it very clear that whilst I expect her to chivvy the kids along to tidy their rooms I do not expect her to follow the kids around tidying up after them.

What I wrote on my APW profile:

"The au pair will need therefore, to prepare some meals for the children and to keep the kitchen tidy after these meals.

To help out with the childrens laundry ...

Somebody needs to make sure the children keep their rooms tidy (but not tidy their rooms for them!)."

Hope that is helpful

It really depends on what you arrange with the individual AP. I always leave scope for them to drop their least favourite chore from my list. Our current AP does: hoovering twice a week, cleans the bathroom, does the laundry and a small amount of ironing, empties the dishwasher every week day, cooks a meal once a week and is expected to help me with any meal preparation on the other days. It works out at about 45 min of housework per working day, the rest of the time is help with childcare.