  Isotope effects in a multiband superconductor with a leading interband
pairing channel are investigated. A relatively small electron-phonon
contribution into the pair-transfer interaction can cause effects of observed
magnitude. A multiband model which interpolates the cuprate properties is used
for illustrative calculations. Isotope exponents of the transition temperature
($\alpha$), supercarrier density ($\alpha_n$), paired carrier effective mass
($\alpha_m$) and of the penetration depth ($\alpha_{\lambda}$) on the doping
scale have been obtained. The known opposite trend of $\alpha$ and $T_c$ is
reflected. The trends of $\alpha$ and $\alpha_n$ are similar. In contrast with
the usual assumption in the interband case $\alpha_{\lambda}$ is driven by
$\alpha_n$, which overwhelmes $\alpha_m$.
