Tenant wants parents to pay rent - how?

Hi,

I have a girl interested in renting my flat. It would be her and her fiance but she says her parents will be paying the rent. How does this work? What kind of agreement would I need with the parents? The tenancy agreement i assume should be for the 2 living in the flat but if neither of them is actually paying the monthly rent. At first i thought the parents would be guarantors but that is only if the tenant fails to pay the rent? I am I right?

Please help.

Thanks