Direct Debit To Credit Card

[chanel5]

I am hoping that some ABers can give me the answer to a banking question.
My sister and her partner have a joint current account, and a number of direct debits come out every month. They also have a credit card each, both of which are paid off monthly from the joint account.
My sister is the main wage-earner, and her partner is unemployed.
My sister has recently discovered that her partner has arranged for a direct debit to be taken from the credit card every month. When she asked her partner about it, the partner insisted it was a private matter. Sister feels (justifiably, I think) that she has a right to know what this payment is for, as sister's salary is paid directly into the joint account.
Sister has seen the credit card statement which just says "direct debit" with no indication of where the payment is going. Account statement just says "payment to card number".
Does anyone know how my sister could find out what this payment is for, as it is a fairly substantial amount?
Thank you in advance to anyone who can offer any advice on how to solve this mystery. Would the bank be likely to agree to tell my sister what the payment is for? We thought they probably wouldn't as the credit card is in the partner's sole name, even though it is paid off from the joint account.

[hc4361]

Yes, it could, ummm. It could also be a CPA.

[Hopkirk]

I suspect he has a hidden debt, and this 'substantial amount' is the minimum monthly payment. Either that, or it goes to a bookies for a gambling addiction.