In the light of the recent WMAP results we update the constraints on a class of non standard BBN models with a simultaneous combination of non standard neutrino distributions and extra effective number of neutrinos in the expansion rate .
These models can be described in terms of the two parameters \Delta N _ { \nu } ^ { tot } , constrained by the primordial Helium abundance Y _ { p } measurement , and \Delta N _ { \nu } ^ { \rho } , constrained by a combination of CMB and primordial Deuterium data .
Small deviations from standard Big Bang Nucleosynthesis are suggested .
Different non standard scenarios can be distinguished by a measurement of the difference \Delta N _ { \nu } ^ { f _ { \nu } } = \Delta N _ { \nu } ^ { tot } - \Delta N _ { \nu } ^ { \rho } .
From the current data we estimate \Delta N _ { \nu } ^ { f _ { \nu } } \simeq - 1.4 ^ { +0.9 } _ { -1.4 } , mildly disfavouring solutions with a low expansion rate , characterized by \Delta N _ { \nu } ^ { f _ { \nu } } = 0 and negative \Delta N _ { \nu } ^ { \rho } .
Active-sterile neutrino mixing could be a viable explanation only for high values of Y _ { p } \gtrsim 0.24 .
The existence of large positive neutrino chemical potentials \xi _ { i } \sim 0.05 , implying \Delta N _ { \nu } ^ { \rho } \simeq 0 , would be a possible explanation of the data within the analyzed class of non standard BBN models .
Interestingly it would also provide a way to evade the cosmological bounds for ‘ class A 3+1 ’ four neutrino mixing models .
A scenario with a decaying sterile neutrino is also considered .