An 85-year-old woman is referred to your cardiology clinic because of an incidental finding of atrial fibrillation at a routine check-up. The patient is asymptomatic from a cardiovascular perspective, but a 24-hour tape organized by the GP shows atrial fibrillation throughout with rates varying between 60 and 110 bpm. The patient has a history of hypertension and stable angina. Coronary angiography performed several years ago showed minor atheroma in the LAD, circumflex, and RCA. Echocardiography shows good biventricular systolic function with a left atrial diameter of 5.2 cm. The patient is on aspirin 75 mg od, ramipril 10 mg od, simvastatin 20 mg od, and atenolol 50 mg od. What thromboprophylactic treatment do you recommend?