The nature of the young and low-mass open clusters Pismis5, vdB80,
  NGC1931 and BDSB96
  We investigate the nature of 4 young and low-mass open clusters (OCs) located
in the $2^{nd}$ and $3^{rd}$ quadrants with near-IR 2MASS photometry (errors
$<0.1$ mag). After field decontamination, the colour-magnitude diagrams (CMDs)
display similar morphologies: a poorly-populated main sequence (MS) and a
dominant fraction of pre-MS (PMS) stars somewhat affected by differential
reddening. Pismis 5, vdB 80 and BDSB 96 have MS ages within $5\pm4$ Myr, while
the MS of NGC 1931 is $10\pm3$ Myr old. However, non-instantaneous star
formation is implied by the wider ($\sim20$ Myr) PMS age spread. The cluster
masses derived from MS + PMS stars are low, within $\sim60-180 \ms$, with mass
functions (MFs) significantly flatter than Salpeter's initial mass function
(IMF). Distances from the Sun are within $1.0-2.4$ kpc, and the visual
absorptions are in the range $\aV=1.0-2.0$. From the stellar radial density
profiles (RDPs), we find that they are small ($\rc\la0.48$ pc, $\rl\la5.8$ pc),
especially Pismis 5 with $\rc\approx0.2$ pc and $\rl\approx1.8$ pc. Except for
the irregular and cuspy inner regions of NGC 1931 and Pismis 5, the stellar
RDPs follow a King-like profile. At $\sim10$ Myr, central cusps - which in old
clusters appear to be related to advanced dynamical evolution - are probably
associated with a star-formation and/or molecular cloud fragmentation effect.
Despite the flat MFs, vdB 80 and BDSB 96 appear to be typical young, low-mass
OCs. NGC 1931 and especially Pismis 5, with irregular RDPs, low cluster mass
and flat MFs, do not appear to be in dynamical equilibrium. Both may be
evolving into OB associations and/or doomed to dissolution in a few $10^7$ yr.
