Mixed first and second class neutrino oscillations

We argue that the general form of the neutrino mass matrix implies that second-class (active→sterile) oscillations require serious consideration. A combination of such oscillations in both vacuum and matter is discussed. For the latter scenario we derive the general mass matrix for three light Majorana neutrinos, one of which is sterile. Implications for solar neutrino observations by neutral-current detectors, such as the Sudbury Neutrino Observatory, are discussed. (orig.)