  We study the impact of Standard Model singlet neutrinos on neutrino flavor
transformation. We focus on an extension of the Zee model which includes
singlet neutrinos, and find that the best limits on the interactions of the
singlet neutrinos come from astrophysical phenomena. Singlet neutrino-electron
scattering will impact both the mattter enhanced flavor transformation
potential as well as detector cross sections. If electron neutrino - singlet
neutrino oscillations are responsible for the solar neutrino anomaly, then the
limit on the singlet neutrino interaction strength is of order of the weak
interaction scale. Zee model modification of nu_tau - e scattering also impacts
solar neutrino transformation, although this interaction is more tightly
constrained.
