Louisa Leung of Thomas Cook Travellers Cheques Ltd, holds euro travellers cheque December 31 in Hong Kong, available in interpayment Visa travellers cheque and Thomas Cook MasterCard travellers cheque to be launched on January 1, 1999. On Friday, the euro will merge the present national currencies of 11 European Union states. Trading in the euro will open at 5 a.m. in Sydney on January 4 (1800 GMT on January 3).