  In this talk we present the update (including the recent SNO results) of our
analysis of the neutrino oscillation solutions of the solar and atmospheric
neutrino problems in the framework of four-neutrino mixing where a sterile
neutrino is added to the three standard ones and the mass spectra present two
separated doublets. Such scenarios allow for simultaneous transitions of solar
$\nu_e$, as well as atmospheric $\nu_\mu$, into a combination of active and
sterile neutrinos. We evaluate the allowed active-sterile admixture in both
solar and atmospheric oscillations from the combined analysis. Our results show
that, although the Super-Kamiokande data disfavour both the pure
$\nu_\mu\to\nu_s$ atmospheric channel and, in combination with SNO, the pure
$\nu_e\to\nu_s$ solar channel, the result from the combined analysis still
favours close-to-pure active and sterile oscillations and disfavours
oscillations into a near-maximal active-sterile admixture.
