  We have investigated the effect of oxygen isotope substitution on the
metal-insulator transition temperature and the resistivity of the narrow band
manganite (La0.25Pr0.75)0.7Ca0.3MnO3 in a constant magnetic field. A set of 16
samples having different mixtures of 16O, 17O and 18O isotopes with average
mass varying from 16.0 to 17.8 a.m.u. was studied. We have found that the
magnetoresistance and the isotope effect can be linked together with a single
parameter - effective magnetic field, which decreases linearly with an increase
of average oxygen mass with a slope of -2 T/a.m.u. The applicability of the
small polaron model is discussed.
