We present an updated analysis of the solar neutrino problem in terms of both
Mikheyev-Smirnov-Wolfenstein (MSW) and vacuum neutrino oscillations, with the
inclusion of the preliminary data collected by the SuperKamiokande experiment
during 306.3 days of operation. In particular, the observed energy spectrum of
the recoil electrons from 8B neutrino scattering is discussed in detail and is
used to constrain the mass-mixing parameter space. It is shown that: 1) the
small mixing MSW solution is preferred over the large mixing one; 2) the vacuum
oscillation solutions are strongly constrained by the energy spectrum
measurement; and 3) the detection of a possible semiannual modulation of the 8B
\nu flux due to vacuum oscillations should require at least one more year of
operation of SuperKamiokande.Comment: 15 pages (RevTeX) + 8 figures (postscript). Requires epsfig.st
