II.23.2 Recovery procedure  II.23.3 Interest on late payment  II.23.4 Recovery rules in the case of joint tender  II.24 CHECKS AND AUDITS  34  II.24.3  The contractor must grant the c ontracting authority’s staff and outside  personnel  authorised  by the contracting authority the appropriate right of access to sites and premises where the  contract is performed and to all the information, including information in electronic format,  II.24.2  The contractor must keep all original documents stored on any appropriate medium, including  digitised originals if authorised under national law, for a period of five years starting from the  payment of the balance. II.24.1  The contracting authority and the European Anti-Fraud Office may check or require an audit  on the  performance of the contract . This may be carried out either by OLAF’s own staff or by  any outside body authorised to do so on its behalf. (c) by taking legal action. (b) by calling in a financial guarantee if the contractor has submitted one to the contracting  authority; (a) by offsetting them against any amounts owed to the contractor by the Union or by the  European Atomic Energy Community or by an executive agency when it implements the  Union budget; Before recovery, the contracting authority must  formally notify the contractor of its intention to  recover the amount it claims, specifying the amount due and the reasons for recovery and inviting the  contractor to make any observations within 30 days of receipt.  If no observations have been submitted or if, despite the observations submitted, the contracting  authority decides to pursue the recovery procedure, it must confirm recovery by  formally notifying  a  debit note to the contractor, specifying the date of payment. The contractor must pay in accordance  with the provisions specified in the debit note.  If the contractor does not pay by the due date, the contracting authority may, after informing the  contractor in writing, recover the amounts due:  If the contractor does not honour the obligation to pay the amount due by the date set by the  contracting authority in the debit note, the amount due bears interest at the rate indicated in  Article II.21.8. Interest on late payments will cover the period starting on the day after the due date  for payment and ending on the date when the contracting authority receives the full amount owed. Any partial payment is first entered against charges and interest on late payment and then against  the principal amount.  If the contract is signed by a group (joint tender), the group is jointly and severally liable under the  conditions set out in Article II.6 (liability). The contracting authority shall send the debit note first to  the leader of the group. If the leader does not pay by the due date the whole amount, and if the amount due cannot be offset  or can only be offset partially in accordance with Article II.23.2 (a), then the contracting authority  may claim the amount still due to any other member or members of the group by respectively  notifying them with a debit note in conformity with the provisions laid down in Article II.23.2. Such checks and audits may be initiated at any moment during the  performance of the contract  and up to five years starting from the payment of the balance. The audit procedure is initiated on the date of receipt of the relevant letter sent by the  contracting authority. Audits are carried out on a confidential basis. 