There are 3 long lived (over 10^9 years) isotopes which may capture electrons and have actually been observed to undergo single beta transmutation. Interestingly, all these 3 ALSO can undergo beta decay AND positron emission. Potassium 40 has a branching ratio of about 1000 electron captures per positron emission. But how about the other two? What are the experimentally observed branching ration between positron emission and electron capture for lanthanum 138 and vanadium 50?