All primordial nuclides are beta decay stable, with the exception of K, V, Rb, Cd, In, La, Lu, and Re. In addition, Te and Ta have not been observed to decay, but are believed to undergo beta decay with an extremely long half-life (over 10 years). (Te can only undergo electron capture to Sb, whereas Ta can decay in both directions, to Hf or W.) Among non-primordial nuclides, there are some other cases of theoretically possible but never-observed beta decay, notably including Rn and Cm (the most stable isotopes of their elements considering all decay modes). Finally, Ca and Zr have not been observed to undergo beta decay (which is theoretically possible for both), but double beta decay is known for both. All elements up to and including nobelium, except technetium and promethium, are known to have at least one beta-stable isotope.