  A relic lepton asymmetry orders of magnitude bigger than the baryon one may
hide in the relic neutrino background. No direct theoretical or experimental
limitations on its magnitude and sign are known. Indirect cosmological
constraints exist ranging from $|L|<0.01$ to $L<10$. We discuss a BBN model
with late electron-sterile neutrino oscillations which is a fine leptometer -
it is capable of feeling extremely small relic lepton asymmetry -
$|L|>10^{-8}$. This sensitivity is achieved through the influence of such small
$L$ on the neutrino oscillations, suppressing or enhancing them, and thus
changing the primordially produced $^4He$. The influence of $L$ on nucleons
freezing in pre-BBN epoch is numerically analyzed in the full range of the
oscillation parameters of the model and for $ L \ge 10^{-10}$. The case of
oscillations generated asymmetry by late electron-sterile oscillations and its
effect on primordial $^4He$ is also briefly discussed.
