  Although the stellar and substellar populations have been studied in various
young and old open clusters, additional studies in clusters in the age range
from 5 to 100 Myr is crucial (e.g. to give more constrains on initial mass
function variation with improved statistics). Among the open cluster candidates
from recent studies, two clusters are best suited for photometric survey of
very-low mass stars and brown dwarfs, considering their youth and relative
proximity: Alessi 5 (t ~ 40 Myr, d ~ 400 pc) and beta Monocerotis (t ~ 9.1 Myr,
d ~ 400 pc). For both clusters, we performed an optical and near-infrared
photometric survey, and a virtual observatory survey. Our survey is predicted
to be sensitive from the massive B main sequence stars down to brown dwarfs of
30 M_Jup. Here, we present and discuss preliminary results, including the mass
function obtained for Alessi 5, which is surprisingly very similar to the mass
function of the Hyades (t ~ 600 Myr), although they are of very different ages.
